The build system defines the IMAGE_BL* macro when compiling each image.
This is useful to distinguish which image the current file is being
built for by using #if defined(IMAGE_BL2) or #if defined(IMAGE_BL31),
or whatever.

There are some cases where we are more interested in which exception
level the current file is being built for.

include/lib/cpus/{aarch32,aarch64}/cpu_macros.S defines IMAGE_AT_EL3,
but we do not have it globally.

Pass IMAGE_AT_EL1 or IMAGE_AT_EL3 to BL*_CFLAGS so that it is available
from all C code.

The library code (libc.a, libmbedtls.a, etc.) is exceptional cases,
where the code can be shared between BL images.

Other than that, we know the exception level at the build time, and
this macro will be useful in the shared code.

Change-Id: I7c8a1da10726906adfba981cfe8464dff111d6b0
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>