build_macros: Add 'add_define_val' macro
build_macros: Add 'add_define_val' macro

Add a convenience macro to add a build definition with a value.

Signed-off-by: Soren Brinkmann <f9793267bd24439886cd4a142f9642a9bcc6364b@xilinx.com>

lsigithub/arm-trusted-firmware_public,lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,sbranden/arm-trusted-firmware,sbranden/arm-trusted-firmware,achingupta/arm-trusted-firmware