fix memory leak in dfs_elm.c and umount fatfs when mount fatfs fails (this bug leads to mkfs failure in linux)

Add RT_MTD_ESRC definitions. If the source page has issue when copying a page, the low level can return this error code. 

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

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

or equivalent to the SConstruct.

Declare sin/cos and M_PI in math.h.

remove product ID

add a dcd operation to send status in control transfer