rootfs_*.bbclass: exclude BUILDNAME var dependency from do_rootfs

rootfs_*.bbclass: exclude BUILDNAME var dependency from do_rootfs

* I have kernel recipe which depends on other recipe to build tiny initramfs
  image, without this change it rebuilds not only that initramfs image
  but also whole kernel when DATE or TIME is changed and OEBasicHash enabled
* also resolves ERROR shown when bitbake -S is used for image:
  ERROR: Bitbake's cached basehash does not match the one we just generated
  (/OE/oe-core/openembedded-core/meta/recipes-core/images/core-image-minimal.bb.do_rootfs)!
  ERROR: The mismatched hashes were 8c35cdf8a5d09c03941f081dd9f6d8dc and b5d6e2e5952770557c48c5779ddb73fc

(From OE-Core rev: a78d2d21fffd646a0d6e0f02a8355be010a7bd28)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>