flex: bump version to 2.6.4

flex: bump version to 2.6.4

- 0001 patch removed. It doesn't apply on this version and is also not
  necessary since the AR binary is now handled correctly with AR = @AR@.
- 0002 patch removed. It's already included in this release:
  https://github.com/westes/flex/commit/a5cbe929ac3255d371e698f62dc256afe7006466
- The flex project has moved to GitHub, so modify the URLs in both
  flex.mk and Config.in files.
- The sed command over Makefile.in to prevent the flex binary to be
  built fails, so remove it. That logic has been moved to
  src/Makefile.in, and whenever we disable the flex binary to be built
  then the compilation fails when building the documentation because
  some bits require "../src/flex", which is the flex binary.
  We prevent building the flex binary and the documentation using a new
  patch.

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