Customize command string in scons

sync with rtgui-0.6

class driver can custom string description

This fix the compiling error:

    /usr/lib64/gcc/x86_64-slackware-linux/4.7.2/../../../../x86_64-slackware-linux/bin/ld: build/components/rtgui/common/dc.o: undefined reference to symbol 'sin@@GLIBC_2.2.5'
    /usr/lib64/gcc/x86_64-slackware-linux/4.7.2/../../../../x86_64-slackware-linux/bin/ld: note: 'sin@@GLIBC_2.2.5' is defined in DSO /lib64/libm.so.6 so try adding it to the linker command line
    /lib64/libm.so.6: could not read symbols: Invalid operation
    collect2: error: ld returned 1 exit status

some bug fix after the recently merge from simmerge branch

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

merge simulator for linux to master branch

fix bug in finsh when built with 64bit-gcc

fixed the string descriptor send to host more than actual