Add two flags for the latest ckati

* --regen

Re-generate build.ninja only when necessary. If either

1. .mk file is updated,
2. environment variable is updated,
3. $(wildcard) result is changed, or
4. $(shell) result is changed,

ckati will regenerate ninja file. This check takes only ~1
second, so incremental build will become much faster even
without "fastincremental" target.

* --ignore_dirty=out/%:

Some .mk files in out/ (e.g.,
out/target/product/generic/previous_build_config.mk)
are updated while ckati is running. With this flag, ckati
does not regenerate build.ninja when they look modified.
This should be OK for ninja based build, as ninja handles
command line changes nicely.

Change-Id: I7a2fca0e327d999599d6b16f06358e8a5e657565