BOOT: Add "bootz" command to boot Linux zImage on ARM

This command boots Linux zImage from where the zImage is loaded to. Passing
initrd and fdt is supported.

Tested on i.MX28 based DENX M28EVK
Tested on PXA270 based Voipac PXA270.

NOTE: This currently only supports ARM, but other architectures can be easily
added by defining bootz_setup().

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Warren <TWarren@nvidia.com>
Cc: albert.u.boot@aribaud.net
Cc: afleming@gmail.com,
Cc: Simon Glass <sjg@chromium.org>,
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>