Build: Refine RTX library selection

1. Only select one basing on CPU/Arch if it's not defined (by platforms)
2. Add checking library existence

Change-Id: Ibd4e5651f8e66173d0b07c881f4b4cba9d4d0216
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
1 file changed