Merge pull request #1 from RT-Thread/master

sync
[GDB stub]first version
[bsp]add gdb support for stm32f4xx
[libcpu]am335x edit vector

vector_undef and vector_dabt
[bsp]add gdb support for BBB
Revert "[bsp]add gdb support for BBB"

This reverts commit a966dd3c3802198cf84adbd8aef08d9206e256ce.
[bsp]add gdb support for BBB
[bsp]stm32f4xx typo
Merge pull request #2 from RT-Thread/master

sync
[bsp]Comment stm32f407 uart6
[gdb]Change open flag
[bsp]change BBB uart flag
Merge pull request #343 from wzyy2/master

[GDB stub]add GDB stub