cmd_bootm.c: Re-order bootm_load_os return check for ELDK4.2

With ELDK4.2 we were getting a warning that load_end may be used
uninitialized in calling lmb_reserve.  This could not be the case,
however.  If we re-order the checks (and make them slightly clearer as
well) the warning goes away.  bootm_load_os may only return 0 on
success, BOOTM_ERR_OVERLAP in a non-fatal overlap (already covered in
comments) or a fatal BOOTM_ERR that is covered in the error handler.

Signed-off-by: Tom Rini <trini@ti.com>