x86: theadorable-x86-common: Remove "ip=dhcp" from default environment

Without ethernet cable plugged, "ip=dhcp" leads to a complete hangup in
Linux booting and the system does not boot into userland at all. Since
its not required to have an active network connection on these board,
lets remove this statement from the default environment.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>