Not only defined for KL25Z as it was by default. I added a Platforms file where are all platforms I found supported in the mbed sources. All you need to do, choose your board.
Supported platforms:
- KL05Z
- KL25Z
- KL46Z
- LPC1768
- LPC11U24
- LPC11U24_301
- LPC1347
- LPC1114
- LPC11C24
- LPC11U35_401
- STM32F407