build-linux.sh: revise config fragments for linux build

Current build-system uses a base defconfig to build .config file for
linux. Though there is a provision to pass additional config
fragments, these are not properly appended to a base defconfig
generated .config and are treated as separate config files altogther.
The existing framework also uses an outdated config fragment path.

Revise and re-introduce fragments framework. Instead of treating
fragments as individual configs, treat them as additions or deletion of
configs from the existing .config built from base defconfig. Make use
of existing build framework to pass one or more config fragments.
Fragments are appended to the .config file generated from defconfig one
at a time. The kernel image is finally built with the appended .config.

Signed-off-by: Rohit Mathew <rohit.mathew@arm.com>
Change-Id: Ib572a591ed6b27a0daf7e50d763e9a5411257df2