Let qemu monitor use stdio and leave window for serial
Reorganize file structure

* src <- Source files
* include <- Header files
* tool <- Tools (gdbscript, shell scripts, mkromfs, ...)
* data <- Data (romfs files)
make: Make it automatically find source code
make: Move rules and variables about romfs to romfs.mk
make: fix paths in qemu rules
make: Move rules and variables about qemu to qemu.mk
make: Move included mk files to ./mk
make: Automatically start gdb
gdbscript: change file source