Make bbappend changing RDEPENDS variables conditional on "agldemo"
AGL_FEATURES flag. This resolved yocto-check-layer errors:
INFO: test_signatures (common.CommonCheckLayer)
INFO: ... FAIL
AssertionError: Adding layer meta-agl-demo changed signatures, basehash changed
Bug-AGL: SPEC-4685
Change-Id: If4953d684455b6f2ab5edbb3a33b5cf7775d1587
Signed-off-by: Denys Dmytriyenko <denys@konsulko.com>