imx6qsabrelite starts to using 'meta-boundary' layer

imx6qsabrelite starts to using 'meta-boundary' layer

a) Adding 'meta-boundary' to bblayers.conf.
b) Prevent of build errors from missing bb files (firefox, chromium),
   with help of BBMASK variable.

Bug-AGL: SPEC-905

Change-Id: Id0b823d4f0e8977206dbd22684e0d2cf9e88465f
Signed-off-by: Mihail Grigorov <michael.grigorov@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/11569
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Matt Porter <mporter@konsulko.com>