am33xx: config.mk: Add support for additional secure boot image types

Depending on the boot media, different images are needed
for secure devices. The build generates u-boot*_HS_* files
as appropriate for the different boot modes.

For AM33xx devices additional image types are needed for
various SPL boot modes as the ROM checks for the name of
the boot mode in the file it loads.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>