Misc kernel config changes

- Changed LED support from module to built-in, so that MMC led works under Berryboot.
- Changed a couple HID modules from module to built-in
Pi: change default CPU governor to ondemand
wait for network link before attempting DHCP
Mount ro first, remount rw in background

Makes Berryboot feel more responsive recovering after unclean shutdown
Add support for 256 MB memsplit

Also no longer mentions ARM memory, but just gpu_mem parameter in memsplit edit dialog
Closes #53