bootm: Reinstate special case for standalone images

For standalone images, bootm had a special case where the OS boot function
was NULL but did actually exist. It was just called manually.

This was removed by commit 35fc84fa which checks for the non-existence of
this function before the special case is examined.

There is no obvious reason why standalone is handled with a special case.
Adjust the code so that standalone has a normal OS boot function. We still
need a special case for when the function returns, but at least we can
avoid the main problem.

This is intended to fix the reported:

    ERROR: booting os 'U-Boot' (17) is not supported

but needs testing.

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