Clarify bootm OS arguments

At present the arguments to bootm are processed in a somewhat confusing
way. Sub-functions must know how many arguments their calling functions
have processed, and the OS boot function must also have this information.
Also it isn't obvious that 'bootm' and 'bootm start' provide arguments in
the same way.

Adjust the code so that arguments are removed from the list before calling
a sub-function. This means that all functions can know that argv[0] is the
first argument of which they need to take notice.

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