layer.conf: fix parse

after meta-systemd commit [1] the following error was thrown during parsing:

ERROR: Unable to parse /home/Superandy/data/oe-core/sources/meta-raspberrypi/conf/layer.conf:
Failure expanding variable BBFILES, expression was
...
/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/recipes-*/*/*.bb
/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/oe-core/recipes-*/*/*.bbappend
/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/meta-oe/recipes-*/*/*.bbappend
${@base_contains('BBFILE_COLLECTIONS', 'systemd-layer','', '/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/invalid.bb', d)}
${@base_contains('BBFILE_COLLECTIONS', 'gnome-layer','/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/meta-gnome/recipes-*/*/*.bbappend', '', d)}
${@base_contains('BBFILE_COLLECTIONS', 'efl-layer','/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/meta-efl/recipes-*/*/*.bbappend', '', d)}
${@base_contains('BBFILE_COLLECTIONS', 'multimedia-layer','/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/meta-multimedia/recipes-*/*/*.bbappend', '', d)}
${@base_contains('BBFILE_COLLECTIONS', 'networking','/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/meta-networking/recipes-*/*/*.bbappend', '', d)}
${@base_contains('BBFILE_COLLECTIONS', 'xfce-layer','/home/Superandy/data/oe-core/sources/meta-openembedded/meta-systemd/meta-xfce/recipes-*/*/*.bbappend', '', d)}
...
which triggered exception NameError: name 'base_contains' is not defined

[1] http://cgit.openembedded.org/meta-openembedded/commit/?id=3c21a46020bd0816579648f684c41dbd6333583e