Merge changes from topic "stm32mp1-multi-image" into integration

* changes:
  stm32mp1: platform.mk: support generating multiple images in one build
  stm32mp1: platform.mk: migrate to implicit rules
  stm32mp1: platform.mk: derive map file name from target name
  stm32mp1: platform.mk: generate linker script with fixed name
  stm32mp1: platform.mk: use PHONY for the appropriate targets