rm48x50: move uart.c to drv_uart.c
rm48x50: small cleanup on context_ccs.asm
rm48x50: remove unused vPortTaskUsesFPU
rm48x50: turn on VFP support

This support Common VFPv2 sub-architecture.
rm48x50: change STMFD/LDMFD to STMDB/LDMIA

VFP instructions only have IA(Increment After)/DB(Decrement Before)
mode. To keep consistency, just change STM/LDM to DB/IA accordingly.
rm48x50: VFP lazy stacking

You need to turn on RT_VFP_LAZY_STACKING in rtconfig.h. By default, RTT
will turn on VFP for all threads and stack all the VFP registers. When
doing lazy stacking, VFP will only be turned on for the thread who
issued VFP instructions.

Currently, if a thread turned on VFP, it cannot be turned off. RTT will
never know what time the thread doesn't need VFP any more. The thread
might could turn off the VFP for it self in proper time.
move libcpu/arm/rm48x50/ to libcpu/arm/cortex-r4