ZEST_CORE_STM32L496RG custom target for Mbed OS.

In your project root directory:

1. Add the custom target to your project:

   mbed add https://github.com/catie-aq/mbed_zest-core-stm32l496rg.git zest-core-stm32l496rg

2. Enable the custom target by adding or overwriting the custom_targets.json at the root of the project:

   cp zest-core-stm32l496rg/custom_targets.json .

3. Compile for the custom target:

   mbed compile --target ZEST_CORE_STM32L496RG --toolchain GCC_ARM