distro_bootcmd: set devtype for dhcp boot

Set $devtype for DHCP boot, which can be handy for the boot.scr
for detection of devtype used (for example, when the same boot.scr is
used for both mmc/dhcp boot):

if test ${devtype} = "dhcp"; then
	...
fi

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>