uboot: Get rid of temporary variables

Without them, the code is simpler.
uboot: Make boot command configurable

U-Boot offers several ways how an OS can be run. For example one can
choose between go, bootm or bootz commands. The bootm command itself has
a handful of variants [1]. This change adds a possibility for the user
to configure which command or variant shall be used.

[1] http://git.denx.de/cgi-bin/gitweb.cgi?p=u-boot.git;a=blob;f=doc/uImage.FIT/command_syntax_extensions.txt;hb=v2014.10