CMake: hardcode mem map name to "application" for Gcc Arm
CMake: hardcode mem map name to "application" for Gcc Arm

We need to fix linker script differently, it should not need APP_TARGET. This is a series of
commits fixing APP_TARGET in our tree. We should not require it.

The linker script preprocessing will be fixed differently. "application" prefix is temporary until
we clean this up completely.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed