common.base: Remove defconfig from linux builds

When we build linux for a specific config, eg, busybox, it was also
building the defconfig. We never use the defconfig build, so drop it and
speed things up.

Change-Id: I0f98582bab09555ca07af0ac16e41a1264625dd6
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>