Build system: add target to build documentation

This patch adds a new target to build the documentation
from the top-level directory. Here is how to use it (from
the top-level source directory):
   $ make doc
Or
   $ make O=build-test doc

To clean all documentation build artefacts, simply call
   $ make clean
Or
   $ make O=build-test clean

Calling 'make' with no arguments will *not* build the
documentation by default.

Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>

Author: Geoffroy Van Cutsem

Makefile

@@ -10,9 +10,10 @@ HV_OUT := $(ROOT_OUT)/hypervisor
 DM_OUT := $(ROOT_OUT)/devicemodel
 TOOLS_OUT := $(ROOT_OUT)/tools
 MISC_OUT := $(ROOT_OUT)/misc
+DOC_OUT := $(ROOT_OUT)/doc
 export TOOLS_OUT
 
-.PHONY: all hypervisor devicemodel tools misc
+.PHONY: all hypervisor devicemodel tools misc doc
 all: hypervisor devicemodel tools misc
 
 hypervisor:
@@ -36,9 +37,13 @@ misc: tools
 	mkdir -p $(MISC_OUT)
 	make -C $(T)/misc OUT_DIR=$(MISC_OUT)
 
+doc:
+	make -C $(T)/doc html BUILDDIR=$(DOC_OUT)
+
 .PHONY: clean
 clean:
 	make -C $(T)/tools OUT_DIR=$(TOOLS_OUT) clean
+	make -C $(T)/doc BUILDDIR=$(DOC_OUT) clean
 	rm -rf $(ROOT_OUT)
 
 .PHONY: install

doc/Makefile

@@ -12,7 +12,7 @@ SPHINXOPTS    ?= -q
 SPHINXBUILD   = sphinx-build
 SPHINXPROJ    = "Project ACRN"
 SOURCEDIR     = .
-BUILDDIR      = _build
+BUILDDIR      ?= _build
 
 DOC_TAG      ?= development
 RELEASE      ?= latest