build-scp: fix unconditional access to mcp binary

Commit ed0fed88 ("build-scp: specify target path for mcp rom binary")
adds a unconditional access to MCP binary files. This is incorrect
as MCP binary is built only for SGI platforms. Fix this by adding
a check for presence of MCP build output folder.

Change-Id: Icf60f0f4c10fbc60cb74bfd533d7dba9c086385f
Reported-by: Asha R <asha.r@arm.com>
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>