tools: mkimage: add support for gpimage format

This patch add support for gpimage format as a preparatory
patch for porting u-boot for keystone2 devices and is
based on omapimage format. It re-uses gph header to store the
size and loadaddr as done in omapimage.c

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Acked-by: Tom Rini <trini@ti.com>