bootm: Split out bootargs environment reading / writing

At present bootm_process_cmdline_env() reads the 'bootargs' variable and
then writes it back afterwards. This is painful for tests, which would
rather use a simple buffer.

It is also useful for zimage to use a buffer, since it does not actually
put the Linux command line in the bootargs variable.

Refactor the existing code into two pieces. One handles reading and
writing the environment variable, as well as allocating a buffer for use
by the rest of the code, which now operates on a buffer.

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