toolchain-buildroot: convert to the package infrastructure

This commit converts the toolchain-buildroot logic to the package
infrastructure. The package is fairly simple as it only defines
BUILDROOT_LIBC, and depends on host-gcc-final to get the overall
internal toolchain build logic started.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>