debug: gdbstub: add kconfig for buffer size

This adds a kconfig to specify the buffer size for GDB packet
I/O. Some architectures may need a bigger buffer for the general
register packet, and we don't want it to overflow our buffer.

This also changes the packet read/write buffer to be allocated
outside of stack. Since the buffer can be large enough that it
won't fit inside the stack.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>