sptool: append cert_tool arguments.

To support secure boot of SP's update cert tool arguments while
generating sp_gen.mk which in turn is consumed by build system.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I2293cee9b7c684c27d387aba18e0294c701fb1cc
2 files changed