initramfs: Encode dependency on KBUILD_BUILD_TIMESTAMP

gen_initramfs.sh has an internal dependency on KBUILD_BUILD_TIMESTAMP
for generating file mtimes that is not exposed to make, so changing
KBUILD_BUILD_TIMESTAMP will not trigger a rebuild of the archive.

Declare the mtime date as a new parameter to gen_initramfs.sh to encode
KBUILD_BUILD_TIMESTAMP in the shell command, thereby making make aware
of the dependency.

It will rebuild if KBUILD_BUILD_TIMESTAMP changes or is newly set/unset.
It will _not_ rebuild if KBUILD_BUILD_TIMESTAMP is unset before and
after. This should be fine for anyone who doesn't care about setting
specific build times in the first place.

Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Tested-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>