Updates to build_api.py (platform_init.c remains platform_init.o in mbed.bld) and moved NMI_Handler from gpio_irq_api.c to platform_init.c for KL05Z, KL25Z and KL46Z target platforms to take advantage of this change.
Merge pull request #230 from sg-/master

Create a platform specific file to override WEAK Handlers and pre-main hook mbed_sdk_init

I thought about a better solution for this problem, but I couldn't find one, so this is fine. Just one comment: I wouldn't name that file 'platform_init.c'. First, it might contain other code than just initialization code (for example, mbed_die is also declared as a weak function and we'll need to override it for NRF51822). Second, it doesn't realy send the "this file is special and should be treated as such" message. I'd call it something like 'mbed_overrides.c'. But I'll make this change later. Thanks for the pull request!