bootm: Split out code from cmd_bootm.c

This file has code in three different categories:
- Command processing
- OS-specific boot code
- Locating images and setting up to boot

Only the first category really belongs in a file called cmd_bootm.c.

Leave the command processing code where it is. Split out the OS-specific
boot code into bootm_os.c. Split out the other code into bootm.c

Header files and extern declarations are tidied but otherwise no code
changes are made, to make it easier to review.

Signed-off-by: Simon Glass <sjg@chromium.org>