bootm: do not hang on failure

bootm: do not hang on failure

On ARMv8 systems

    load mmc 0:1 $loadaddr  vmlinuz-5.10.0-3-arm64
    booti

leads to a hanging system requiring to physically reset the system:

    FDT and ATAGS support not compiled in - hanging
    ### ERROR ### Please RESET the board ###

For systems where physical access is difficult hanging is a poor choice.
It is preferable to reset the system when U-Boot reaches a state that is
not recoverable.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>