build(rzg-layout): split combined targets

This is a small change to split up the generation of the RZ/G layout
images into unique targets. This is predominantly for cleanliness
reasons - Make current doesn't know about the `.bin` and `.srec`
binaries generated by the `.elf` target.

Change-Id: I81251ac647b85c5eec8f910ddc841a5a32b49e67
Signed-off-by: Chris Kay <chris.kay@arm.com>
2 files changed