Fix TAGS creation

Fix TAGS creation

Currently the Makefile creates TAGS for emacs with the command:
    find "$(SRC_PATH)" -name '*.[hc]' -print0 | xargs -0 etags
That works only if xargs ends up invoking etags just once.  If xargs runs
etags several times, as it will if there are enough files, then the later
invocations will overwrite the output from the earlier invocations.  This
patch uses the etags --append option to fix the bug.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Message-id: 1363057048-21534-1-git-send-email-david@gibson.dropbear.id.au
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>