board: efm32pg_stk3402a: Add option to compile for EFM32JG12B

By default the firmware is compiled for the EFM32PG12B SoC with the ARM
Cortex-M4F core. When building for efm32pg_stk3402a_jg, The firmware is
compiled for the EFM32JG12B SoC with the ARM Cortex-M3 core.

Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>
9 files changed