BaseTools/VfrCompile: honor EXTRA_LDFLAGS

BaseTools/VfrCompile: honor EXTRA_LDFLAGS

In commit 81502cee20ac ("BaseTools/Source/C: take EXTRA_LDFLAGS from the
caller", 2018-08-16), I missed that "VfrCompile/GNUmakefile" does not use
BUILD_LFLAGS in the APPLICATION linking rule, unlike "app.makefile" does.
Instead, "VfrCompile/GNUmakefile" uses the (undefined) LFLAGS macro.
Therefore commit 81502cee20ac did not cover the linking step of
VfrCompile.

Thankfully, the structure of the linking rules is the same, between
"app.makefile" and "VfrCompile/GNUmakefile". Rename the undefined LFLAGS
macro in "VfrCompile/GNUmakefile" to VFR_LFLAGS (for consistency with
VFR_CXXFLAGS), and set it to EXTRA_LDFLAGS.

As a result, we have:

             | compilation                    | linking
  -----------+--------------------------------+----------------------
  VfrCompile | VFR_CXXFLAGS =                 | VFR_LFLAGS =
             | BUILD_OPTFLAGS =               | EXTRA_LDFLAGS
             | '-O2' + EXTRA_OPTFLAGS         |
  -----------+--------------------------------+----------------------
  other apps | BUILD_CFLAGS/BUILD_CXXFLAGS =  | BUILD_LFLAGS =
             | [...] + BUILD_OPTFLAGS =       | [...] + EXTRA_LDFLAGS
             | [...] + '-O2' + EXTRA_OPTFLAGS |

This table shows
- that the VfrCompile compilation and linking flags are always a subset of
  the corresponding flags used by the other apps,
- and that the EXTRA flags are always at the end.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1540244
Fixes: 81502cee20ac4046f08bb4aec754c7091c8808dc
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>