Move all command code into its own directory

There are a lot of unrelated files in common, including all of the commands.
Moving them into their own directory makes them easier to find and is more
logical.

Some commands include non-command code, such as cmd_scsi.c. This should be
sorted out at some point so that the function can be enabled with or without
the associated command.

Unfortunately, with m68k I get this error:

m68k:  +   M5329AFEE
+arch/m68k/cpu/mcf532x/start.o: In function `_start':
+arch/m68k/cpu/mcf532x/start.S:159:(.text+0x452): relocation truncated to fit: R_68K_PC16 against symbol `board_init_f' defined in .text.board_init_f section in common/built-in.o

I hope someone can shed some light on what this means. I hope it isn't
depending on the position of code in the image.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>