asciidoc: use absolute path for resources

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>