populate_sdk_ext: copy BBMULTICONFIG files

As the generated local.conf includes BBMULTICONFIG, the referenced files
in conf/multiconfig also need to be copied.  Otherwise with
BBMULTICONFIG="abc" for instance, building the esdk fails with:

ERROR: ParseError at tmp/build-glibc/work/qemux86_64-oe-linux/core-image-ssh/1.0-r0/sdk-ext/image/tmp-renamed-sdk/layers/openembedded-core/meta/conf/bitbake.conf:767: Could not include required file conf/multiconfig/abc.conf

Signed-off-by: Justin Bronder <jsbronder@cold-front.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>