yocto-kernel: enforce exact match for BBLAYERS

yocto-kernel: enforce exact match for BBLAYERS

The current code uses .startswith to find BBLAYERS, which causes false
positives when other variables such as BBLAYERS_NON_REMOVABLE exist.

This forces an exact match instead of a partial match.

Fixes [YOCTO #4743].

(From meta-yocto rev: c039def50ca6c02cb1b66fd4bf76664de42c068e)

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>