package/syslinux: disable documentation

Disable documentation to avoid the following build failure if a non
working asciidoc or a2x is found on the system:

asciidoc -o html/syslinux.html /nvmedata/autobuild/instance-3/output-1/build/syslinux-6.03/txt/syslinux.txt
/nvmedata/autobuild/instance-3/output-1/per-package/syslinux/host/bin/python3: No module named asciidoc

Setting {ASCIIDOC,A2X_XML}_OK to a value different of 0 will disable
html, man, xhtml and text documentation

Fixes:
 - http://autobuild.buildroot.org/results/47f876ccb56831cc1bb9e6c2f7dbce423581a0dd

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 375f5dd1bd036fd8213bade31881b368d2650603)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/syslinux/syslinux.mk

@@ -58,6 +58,8 @@ SYSLINUX_POST_PATCH_HOOKS += SYSLINUX_CLEANUP
 # be used.
 define SYSLINUX_BUILD_CMDS
 	$(TARGET_MAKE_ENV) $(MAKE1) \
+		ASCIIDOC_OK=-1 \
+		A2X_XML_OK=-1 \
 		CC="$(TARGET_CC)" \
 		LD="$(TARGET_LD)" \
 		OBJCOPY="$(TARGET_OBJCOPY)" \