allows the user to custom PID and VID;
add a dcd operation to send status in control transfer
move the define related to Mass to mstorage.h
remove product ID
allows the user custom PID and VID
move the define related to Mass to here;
remove product ID
use new interface to send status in control transfer
allows the user custom PID and VID
revert the define
remove math code for newlib

Newlib already have optimized and full featured math implementation. To
use it, one should add:

    env['LIBS'] = ['m']

or equivalent to the SConstruct.
Merge pull request #9 from heyuanjie87/usb-device

Usb-device
revert some define to usb_common.h
Correct macro definition
Merge pull request #12 from heyuanjie87/usb-device

Usb device:correct the macro
Newlib: link with libm in default.

libm is a frequently used lib. Newlib is compiled with
-ffunction-sections in recent GCC tool chains. The linker would just
link in the functions that have been referenced. So setting this won't
result in bigger text size.
newlib: use the LIBS argument in DefineGroup instead of modifying the Env
add libcpu/sim/posix/cpu_port.c
add bsp: simluator for linux
add some comments and clean code in cpu_port.c
re-write the rt_hw_interrupt_enable/disable
Merge pull request #11 from grissiom/remove-newlib-math

remove math code for newlib