fix(plat/st): add STM32IMAGE_SRC
fix(plat/st): add STM32IMAGE_SRC

The dependency on this macro was added by patch [1]. But the macro
itself was forgotten in the patch.

 [1] 128e0b3e2e0 ("stm32mp1: update rules for stm32image tool")

Change-Id: I49219e1e13828b97b95f404983da33ef4567fe23
Signed-off-by: Yann Gautier <e753f6969c288905606e3c07464886d4f1fc1be0@foss.st.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware