bitbake: include varflags in checksums

bitbake: include varflags in checksums

Add a dependency to each variable on a filtered list of its varflags.
This is intended to catch things such as SRC_URI checksums, varflags
controlling extra functionality from classes (e.g. the recently updated
update-alternatives class in OE-Core), etc. and ensure their values
influence the sstate checksums.

There is an exclusion list which needs to be set via bitbake.conf
(BB_SIGNATURE_EXCLUDE_FLAGS), if this is not set then the functionality
is disabled. The existing vardepsexclude mechanism can also be used to
exclude undesired varflags, but they must be fully specified, e.g.:

do_patch[vardepsexclude] += "do_patch[someflag]"

Implements [YOCTO #2517].

(Bitbake rev: 56c1ab18fcaf4ac245dcb412ed55e8e0af07883b)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>