Change layer name

Conforming to OE/Yocto naming convention, the layer should not use the
meta prefix for BBFILE_COLLECTIONS and other layer-specific variables.

Signed-off-by: David Vincent <freesilicon@gmail.com>
Remove superfluous EXTRA_IMAGE_DEPENDS

Since files/bootloaders.inc is now required by every machine
configuration file, the EXTRA_IMAGE_DEPENDS found in sama5d3xek.conf
is not needed anymore.

Signed-off-by: David Vincent <freesilicon@gmail.com>