Rename the output directories

In the output directory, we now have

 - build/    where all the packages are built
 - images/   where the final kernel and rootfs images are stored
 - staging/  the staging directory (containing the development files
             and libraries compiled for the target)
 - target/   which contains the target root filesystem
 - host/     which contains all the host programs
 - stamps/   which contains the stamps files

Therefore, the build_ARCH and toolchain_build_ARCH have been
removed. People willing to use the same Buildroot sources to compile
for different architectures are invited to use the O= command line
option for out-of-tree compilation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>