Support PLATFORM=<platform>-<flavor>

When $(PLATFORM) contains a hyphen, parse it as
$(PLATFORM)-$(PLATFORM_FLAVOR). Then, one can use:
$ make PLATFORM=vexpress-fvp ...
instead of:
$ make PLATFORM=vexpress PLATFORM_FLAVOR=fvp ...

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>