kconfig: fix update-po-config

Massimo Maiurana <maiurana@gmail.com> reported that
update-po-config was broken:
1) spelling errors in Makefile so arch/um failed
2) UTF-8 was not supported

The following patch address the above problems.
kxgettext now append the output to the .pot file
generated by xgettext - so we have a header.
In all places UFT-8 is specifed so we now flawlessly
support UTF-8.
The Kconfig files had an empty string in a few cases -
these are now supressed in kxgettext.

With this the translators can now pick up where they left
and get it all translated.
There are ~11000 strings to be translated...

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc:  Massimo Maiurana <maiurana@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>