link with libm.so

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
rtgui_demo: port to new API
simulator: no need to dump the bin file
fix compile errors with vs in bsp/simulator
clean code in  bsp/simulator/rtconfig.py
class driver can custom string description
Merge pull request #21 from heyuanjie87/ForPullRequest

class driver can custom string description
show volume size in KBytes in df
Merge branch 'master' of github.com:RT-Thread/rt-thread
create disk/{ad, nor, nand in bsp/simulator
enabe dfs_win32 in simulator when built on windows
sync with rtgui-0.6
Merge pull request #22 from grissiom/rtgui-0.6

sync with rtgui-0.6
Merge branch 'simdev'