build: allow platform makefiles to configure `ENABLE_LTO`

This change introduces a lazily-evaluated condition on `ENABLE_LTO` to
the `LTO_CFLAGS` variable as opposed to evaluating the condition
eagerly.

This concludes a recent request on the mailing list:

    https://lists.trustedfirmware.org/archives/list/tf-a@lists.trustedfirmware.org/thread/EU3XR4VB3RP2NQB372QPZ4VRP57ANNLC/

Change-Id: Ie1f73352eb51fb2ceb2385232336312216ef87fc
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed