Ability to modify distro boot filename

Add in the ability to modify the distro boot filename. Whilst not
immediately useful in normal usage, it allows an alternative
configuration to be provided when other u-boot functionality is used, such
as bootcount limit, to fallback to an alternative boot configuration. In
this case we can follow the same boot path as for normal boot, just
using an alternatively named configuration file.

For example, by providing the following `altbootcmd` when bootcount is in
use:

altbootcmd=setenv boot_extlinx_conf extlinux-rollback.conf; \
	run distro_bootcmd

Signed-off-by: Martyn Welch <martyn.welch@collabora.co.uk>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>