Add support to link an external lib with ARM TF

This patch defines the variable `LDLIBS` which allows external
libraries to be specified to 'ld' to enable it to link the
libraries.

Change-Id: I02a490eca1074063d00153ccb0ee974ef8859a0e
Signed-off-by: Soby Mathew <soby.mathew@arm.com>