build-scp: align to changes in scp firmware binary names

The SCP firmware binary names are now changed to scp_romfw.bin,
scp_ramfw.bin, mcp_romfw.bin and mcp_ramfw.bin. Align to these
changes in the scripts that operate the SCP/MCP firmware binary
images.

Change-Id: Ie0171da2de3bdd71f632d9e1a218adda1c707524
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>