mach-omap2: Fix secure boot media generation

While moving OMAP related files to mach-omap2 the functionality
relating to generating secure boot files was modified. This change
prevents secure platforms other than AM33xx and OMAP54XX from
correctly building files for all needed media types.

Fixes: 983e3700 ("arm: Introduce arch/arm/mach-omap2 for OMAP2 derivative platforms")
Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>