uboot-envtools: check for config prior to append

uboot-envtools: check for config prior to append

In the rare event a pre-populated fw_env.config exists in the rootfs prior
to firstboot, calling fw_setenv after the system has initialised will
annihilate the devices environment due to two identical lines in
fw_env.config.

Check for existence prior to blind appendage.

Signed-off-by: Imran Khan <gururug@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[daniel@makrotopia.org: fixed patch format, improved grep, cosmetics]