build/infra: add a defconfig for buildroot component

Buildroot works on defconfig against which the packages are built. For
each platform, add a defconfig that can be copied to buildroot
directory to allow the buildroot sources to be built.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: Ibfb44a7fbdb008547fe0eae6c415bdc6e200b07b