cortex-r4: add __rt_ffs
cortex-r4: wrap asm functions with .asmfunc/.endasmfunc
rm48x50: add cache_{enable, disable}
RM48 does not have cache implemented
rm48x50: rt_interrupt_nest should be `volatile rt_uint8_t`
Merge pull request #109 from grissiom/rm48x50

Rm48x50
tools: add an option to generate cscope database

When --cscope option is given to Scons, it will generate a cscope
cross-reference database in current directory, which is useful in
Vim(and other cscope-aware text editors). For example, `scons -s
--cscope` will do nothing except generating the database. You can use
this option with other options together.

It is inspired by the `make cscope` of Linux.
Merge pull request #110 from grissiom/cscope

tools: add an option to generate cscope database
update libcpu/arm/cortex-m4: restore MSP.
update libcpu/arm/cortex-m3: restore MSP.
update libcpu/arm/cortex-m0: restore MSP.