asciidoc: use absolute path for resources

One of the "features" of a2x is that the '-r|--resource' flag imports
resources relative to the [SOURCE_FILE], not the CWD. The current
implementation tries to import resources from the destination dir $(@D),
which works fine for out-of-tree builds because $(@D) is an absolute
path, however in-tree builds treat $(@D) as a relative path, which breaks
because a2x treats $(@D) relative to SOURCE_FILE.

Use the make command $(abspath names...) to convert the dest dir to an
absolute path before passing it as a resource.

Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Atul Singh Mandla <atul.singh.mandla@rockwellcollins.com>
Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/doc-asciidoc.mk

@@ -117,7 +117,8 @@ $$(O)/docs/$(1)/$(1).$(6): $$($(2)_SOURCES) \
 	$$(Q)$$(call MESSAGE,"Generating $(7) $(1)...")
 	$$(Q)mkdir -p $$(@D)
 	$$(Q)a2x $(8) -f $(4) -d book -L \
-		$$(foreach r,$$($(2)_RESOURCES),-r $$(r)) -r $$(@D) \
+		$$(foreach r,$$($(2)_RESOURCES) $$(@D), \
+			--resource="$$(abspath $$(r))") \
 		$$($(2)_$(4)_A2X_OPTS) \
 		--asciidoc-opts="$$($(2)_$(4)_ASCIIDOC_OPTS)" \
 		$$(BUILD_DIR)/docs/$(1)/$(1).txt