Sorry, this:

$ staging_dir_mipsel/biel-linux-uclibc-gdb build_mipsel/linux-2.6-ar7/linux-2.6.21.5/vmlinux

Is meant to be:

$ staging_dir_mipsel/bin/mipsel-linux-uclibc-gdb build_mipsel/linux-2.6-ar7/linux-2.6.21.5/vmlinux

Cut'n'pasto.