bootm: Reduce arguments to boot_get_kernel()

bootm: Reduce arguments to boot_get_kernel()

This function only uses one argument and just needs to know the name of
the command which called it. Adjust the function to use only what it
needs. This will make it easier to call from a non-command context.

Tidy up the function comment while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>