arm: ti: Fix regression in distro boot for mmc

When devnum was changed to a local variable in distro_bootcmd we ran
into a problem on TI platforms (confirmed on Beaglebone) as we had been
using 'setenv devnum' there as well and it needs to match the other
usage.

Fixes: 13dd6665 ("distro: not taint environment variables if possible")
[trini: Review other platforms, re-word commit message]
Signed-off-by: Tom Rini <trini@konsulko.com>