arm: novena: Switch novena to config_distro_bootcmd

Switch Novena to distro bootcmd, so it can be used with debian easily.

Signed-off-by: Vagrant Cascadian <vagrant@aikidev.net>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Sean Cross <xobs@kosagi.com>
Cc: Stefano Babic <sbabic@denx.de>
Tested-by: Vagrant Cascadian <vagrant@aikidev.net>