toolchain-external: fix getent installation with Codescape MIPS toolchains

toolchain-external: fix getent installation with Codescape MIPS toolchains

When a toolchain is glibc based, the getent package assumes that
$(STAGING_DIR)/usr/bin contains the getent program. Unfortunately, the
Codescape MIPS toolchains do not conform with this:
$(STAGING_DIR)/usr/{bin,sbin} are empty, and instead three directories
are provided: bin-o32, bin-n32 and bin-n64 (ditto for sbin), one for
each supported MIPS ABI.

Since this is a toolchain-specific oddity, we handle it by adding a
post-install fixup hook that creates $(STAGING_DIR)/usr/{bin,sbin} as
symbolic link to the appropriate directory.

Fixes:

   http://autobuild.buildroot.org/results/9c0ee836021553319f166f9de88750535aee0a58/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>