of: add config option to enable building of all dtbs

Enable building all dtb files when CONFIG_OF_ALL_DTBS is enabled. The dtbs
are not really dependent on a platform being enabled or any other kernel
config, so for testing coverage it is convenient to build all of the dtbs.

In order to only build dtbs, this option can be used by creating an
allno.config file containing:
CONFIG_COMPILE_TEST=y
CONFIG_OF=y
CONFIG_OF_ALL_DTBS=y

And then running:
make KCONFIG_ALLCONFIG=1 allnoconfig
make dtbs

While building the dtbs themselves don't need a cross compiler, the
scripts dependency does need one. This can be hacked around by
commenting out "subdir-y += mod" in scripts/Makefile.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Grant Likely <grant.likely@linaro.org>