buildroot

binutils, gdb: support unified binutils-gdb git repository

If Binutils and/or GDB are fetched from the unified binutils-gdb
repository, then the tarball will contain both Binutils and GDB
sources, unlike the "normal" tarballs that contain only the titular
package. To keep packages separated in Buildroot we need to disable
undesired components when configuring.

Binutils and GDB migrated to a common Git repository in the October
2013 [1]. Previous Git repositories were incomplete copies of CVS
repository which copied only the relevant files (no binutils files in
GDB, and vice versa). In the new binutils-gdb repository there is no
such separation and a result all files exist in directory after
checkout. So if "configure" and "make" are used without explicit
targets, all projects will be built: binutils, ld, gas, bfd, opcodes,
gdb, etc. In case of Buildroot this would mean that selecting Binutils
only, still will build both Binutils and GDB.  And if GDB is selected
as well, then both packages will be built two times, and Binutils from
GDB directory will overwrite initial build of Binutils (or vice versa
if Binutils will be built after the GDB). This is a serious problem,
because binutils and GDB use separate branches in this common
repository. In case of Buildroot this means that separate Git commits
(or tags) should be used when downloading source from Git.

This affects only Git repositories, because GNU release tarballs still
contain only relevant packages.

This change is backward compatible, because if "normal" tarball is
used (without extra directories), than --disable-* configure options
are just ignored by configure.

[1] https://sourceware.org/ml/gdb/2013-10/msg00071.html

[Thomas: use variables to factorize options, and add comments in the
relevant .mk files to explain what's going on.]

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>