package/frr: set FRR_XREF_NO_NOTE

Set FRR_XREF_NO_NOTE to avoid the following build failures with
microblazeel, nds32 and xtensa raised since bump to version 8.1 in
commit ca2753fd:

_clippy.ELFAccessError: virtual address (538157256) not found in program headers

[...]

/tmp/ccFGv94v.s:13: Error: can't resolve `__start_xref_array' {*UND* section} - `L0' {.note.FRR section}
/tmp/ccFGv94v.s:14: Error: can't resolve `__stop_xref_array' {*UND* section} - `L0' {.note.FRR section}

Here is an extract of
https://github.com/FRRouting/frr/blob/7347a4859d4b984cea0aef769a16622d3f02e44f/lib/xref.h:
"the following blurb emits an ELF note indicating start and end of the xref
array in the binary.  This is technically the "correct" entry point for
external tools reading xrefs out of an ELF shared library or executable.
...
FRR itself does not need this note to operate correctly, so if you have
some build issue with it just add -DFRR_XREF_NO_NOTE to your build flags
to disable it."

In other words, this is only *possibly* needed for another package that
wants to extracts the xrefs. We currently don't have anything that
depends on frr in-tree, and it's not even installed in staging, so it's
hard to check, but it seems pretty unlikely that this is really needed.

Fixes:
 - http://autobuild.buildroot.org/results/3cdb50f5e5a1b3f37a6edcd4276fcbf015e28828
 - http://autobuild.buildroot.org/results/a3cc0b5090a1faa2bca9c8dfe0fec9b6a918ba4d
 - http://autobuild.buildroot.org/results/694cc65478a82ec93d2074252892036855cdc49d



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>