make po in handbook doesn't work

[Hoyoung Kim]

Hello :)

I recently resumed Korean translation and got in trouble.  I followed
the instruction in 12.3. of
fdp-primer(
http://www.freebsd.org/doc/en_US.ISO8859-1/books/fdp-primer/po-translations-creating.html
)
to generate PO files.  This worked fine with some articles(bsdl-gpl,
contributing and explaining-bsd) and books(developers-handbook, faq,
fdp-primer and porters-handbook).

However, I got the error when I tried to generate po for handbook:

hwong at TeamAFK:~/freebsd-doc-kr/ko_KR.UTF-8/books/handbook % make po
make: don't know how to make
/usr/home/hwong/freebsd-doc-kr/en_US.ISO8859-1/books/handbook/mirrors.lastmod.inc.
Stop

I looked around for several files, but failed to fix.  Makefile of
handbook and current structure of Korean directory can be found here:

https://github.com/kr-bsd/freebsd-doc-kr/blob/master/ko_KR.UTF-8/books/handbook/Makefile

What should I do to create po file for handbook?
Thank you, and Happy New Year!