Rm48x50

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.

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

This support Common VFPv2 sub-architecture.

For pull request

use __rt_ffs instead of ffs

Use condition flag in the ORR. This could eliminate a BEQ.

When saving thread registers in context_switch_interrupt_to, we don't
change them, just move them. So there is no need to always r0-r3 from
stack to the real r0-r3. So just use the intermediate registers and
eliminate 2 MOV.

It does not satisfy AAPCS.

The code is GPLv2 so I think we could use it for free(both free beer and
free speech).

Substitude STMFD, MOV, ADD with STMFD, SUB. It reduce one instruction.
Tested on board and it works like a charm.

use __rt_ffs instead of ffs

usbdevice/core: add reset function to support hotplug