debug: tweak debug device configurations

1. remove source when no debug device exists
2. tweak function prototype

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
debug: add function to change debug device

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
debug: add a simple ram debug device implementation

Signed-off-by: Viller Hsiao <villerhsiao@gmail.com>
Merge pull request #59 from vh21/tweak-dbg

Tweak debug device implementation
slightly tweak licensing terms and style
Merge pull request #58 from georgekang/add-strcmp

lib: string: add strcmp
Add definitions of SYSCFG registers and memory remap function
In debug_uart.c, only include softirq.h with CONFIG_KDB
Correct the type(uint32_t --> size_t) of "size" in function queue_init.
Loader merged with f9 kernel(containing user space)

It is WIP version of loader.  The following are future work.
   1. Merged build system with f9 kernel.
   2. Remove duplicated files in loader(xxx_ld.c).  They are just
      for easy building.