meta-mel/conf/layer.conf: search for *.bb files

Freescale SoC specific applications e.g. skmm-ep require header
files that are only available through linux-qoriq-sdk-headers.
linux-qoriq-sdk-headers.bb is already there but this commit relaxes
search so that *.bb also get picked.

Signed-off-by: Fahad Arslan <Fahad_Arslan@mentor.com>