lsxl: switch from bootm to bootz for boot commands

Use the bootz command to load zImages in case of any new boot scripts. Only
the legacy one will still use bootm. Apart form the fact, that this will
simplify the image generation process, it saves one copy of the kernel
image: Common practice is to generate an uImage with a loading address of
0x8000. This uImage contains a compressed zImage, which will unpack the
kernel image to the beginning of the RAM. But because there is already the
compressed image the uncompressor first relocates the compressed image to a
higher location. The load address is encoded into the uImage which is
generated by the distributions and thus cannot be easily changed. By using
the bootz command we can load the compressed image to a higher memory
address and the decompressor doesn't have to reloacte the image.

Signed-off-by: Michael Walle <michael@walle.cc>