config_distro_bootcmd.h: Use a private variable for bootpart

Hush has an oddity where using ${var} causes var to resolved in the the global
address space (iotw the environment) first and only afterwards will the local
variable space be searched.

This causes odd side-effects when iterating over the boot partitions
using ${bootpart} if the environment also has a bootpart variable (e.g. for
the various TI boards). Fix this by using the hopefully more unique
${distro_bootpart} instead of ${bootpart}.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>