Fix compilation error due to the undefined macro PLATFORM_STM32_STM_DISCO

The undefined macro PLATFORM_STM32_STM_DISCO is not available. This causes
compilation error when some kernel configs (CONFIG_STM32_ETHER...) are enabled.
Replacing PLATFORM_STM32_STM_DISCO with PLATFORM_STM32_STM32F429_DISCO can fix
the issue.