feat(intel): add build option for boot source

Existing boot source is hardcoded in socfpga_plat_def.h.
To change boot source, user need to update code.
Thus adding this will remove the code update needed when
need to change boot source.

Also, it will have ARM_LINUX_KERNEL_AS_BL33 flag for each
platform in platform.mk. This will be easily to control
based on platform build.

Change-Id: I383beb8cbca5ec0f247221ad42796554adc3daae
Signed-off-by: Jit Loon Lim <jit.loon.lim@intel.com>
Signed-off-by: Sieu Mun Tang <sieu.mun.tang@intel.com>
10 files changed