asciidoc: set CLEANBROKEN to fix rebuild

asciidoc: set CLEANBROKEN to fix rebuild

The make clean removes doc/a2x.1 and doc/asciidoc.1, then it would cause
build failures since in the second build:
Fixing CONF_DIR in asciidoc.py
Fixing CONF_DIR in a2x.py
python a2x.py -f manpage doc/a2x.1.txt
a2x: ERROR: "xmllint" --nonet --noout --valid "/path/to/asciidoc-native/8.6.9-r0/asciidoc-8.6.9/doc/a2x.1.xml" returned non-zero exit status 4
make: *** [doc/a2x.1] Error 1

The xmllint failed because "--nonext" is used:
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
/buildarea/lyang1/test_arm/tmp/work/x86_64-linux/asciidoc-native/8.6.9-r0/asciidoc-8.6.9/doc/a2x.1.xml:2: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"

Avoid running make clean will fix the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>