bootm: Factor out common parts of image decompression code

bootm: Factor out common parts of image decompression code

Adjust the code so that the error reporting can all be done at the end,
and is the same for each decompression method. Try to detect when
decompression fails due to lack of space. Keep the behaviour of
resetting on failure even though there should be no memory corruption
now.

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