ktest: Update documentation on config_bisect

#

#  The way it works is this:

#

#   First it finds a config to work with. Since a different version, or

#   MIN_CONFIG may cause different dependecies, it must run through this

#   preparation.

#   You can specify a good config with CONFIG_BISECT_GOOD, otherwise it

#   will use the MIN_CONFIG, and if that's not specified, it will use

#   the config that comes with "make defconfig".

#

#   Overwrites any config set in the bad config with a config set in

#   either the MIN_CONFIG or ADD_CONFIG. Thus, make sure these configs

#   are minimal and do not disable configs you want to test:

#   (ie.  # CONFIG_FOO is not set).

#   It runs both the good and bad configs through a make oldconfig to

#   make sure that they are set up for the kernel that is checked out.

#

#   An oldconfig is run on the bad config and any new config that

#   appears will be added to the configs to test.

#   It then reads the configs that are set, as well as the ones that are

#   not set for both the good and bad configs, and then compares them.

#   It will set half of the good configs within the bad config (note,

#   "set" means to make the bad config match the good config, a config

#   in the good config that is off, will be turned off in the bad

#   config. That is considered a "set").

#

#   Finally, it generates a config with the above result and runs it

#   again through make oldconfig to produce a config that should be

#   satisfied by kconfig.

#   It tests this new config and if it works, it becomes the new good

#   config, otherwise it becomes the new bad config. It continues this

#   process until there's only one config left and it will report that

#   config.

#

#   Then it starts the bisect.

#   The "bad config" can also be a config that is needed to boot but was

#   disabled because it depended on something that wasn't set.

#

#   The configs to test are cut in half. If all the configs in this

#   half depend on a config in the other half, then the other half

#   is tested instead. If no configs are enabled by either half, then

#   this means a circular dependency exists and the test fails.

#   During this process, it saves the current good and bad configs in

#   ${TMP_DIR}/good_config and ${TMP_DIR}/bad_config respectively.

#   If you stop the test, you can copy them to a new location to

#   reuse them again.

#

#   A config is created with the test half, and the bisect test is run.

#

#   If the bisect succeeds, then all configs in the generated config

#   are removed from the configs to test and added to the configs that

#   will be enabled for all builds (they will be enabled, but not be part

#   of the configs to examine).

#

#   If the bisect fails, then all test configs that were not enabled by

#   the config file are removed from the test. These configs will not

#   be enabled in future tests. Since current config failed, we consider

#   this to be a subset of the config that we started with.

#

#   When we are down to one config, it is considered the bad config.

#

#   Note, the config chosen may not be the true bad config. Due to

#   dependencies and selections of the kbuild system, mulitple

#   configs may be needed to cause a failure. If you disable the

#   config that was found and restart the test, if the test fails

#   again, it is recommended to rerun the config_bisect with a new

#   bad config without the found config enabled.

#   Although the MIN_CONFIG may be the config it starts with, the

#   MIN_CONFIG is ignored.

#

#  The option BUILD_TYPE will be ignored.

#

# CONFIG_BISECT_GOOD (optional)

#  If you have a good config to start with, then you

#  can specify it with CONFIG_BISECT_GOOD. Otherwise

#  the MIN_CONFIG is the base.

#  the MIN_CONFIG is the base, if MIN_CONFIG is not set

#  It will build a config with "make defconfig"

#

# CONFIG_BISECT_CHECK (optional)

#  Set this to 1 if you want to confirm that the config ktest

#  generates (the bad config with the min config) is still bad.

#  It may be that the min config fixes what broke the bad config

#  and the test will not return a result.

#  Set it to "good" to test only the good config and set it

#  to "bad" to only test the bad config.

#

# Example:

#   TEST_START