stm32mp1: platform.mk: derive map file name from target name

Doing this allows us in the next commit to use implicit rules (%-patterns)
to cover all the images we generate during a stm32mp1 build.

Change-Id: Ibde59d10ccce42566f82820117d7fd0d77345e6c
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
1 file changed