doc: Replace DocBook with sphinx-based docs

The Linux kernel moved to sphinx-based documentation and got rid of the
DocBook based documentation quite a while ago. Hence, the DocBook
documentation for U-Boot should be converted as well.

To achieve this, import the necessary files from Linux v4.17, and
convert the current DocBook documentation (three files altogether) to
sphinx/reStructuredText.

For now, all old DocBook documentation was merged into a single
handbook, tentatively named "U-Boot Hacker Manual".

For some source files, the documentation style was changed to comply
with kernel-doc; no functional changes were applied.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/Makefile b/Makefile
index 4b3023b..d1246e0 100644
--- a/Makefile
+++ b/Makefile
@@ -1707,9 +1707,11 @@
 
 # Documentation targets
 # ---------------------------------------------------------------------------
-%docs: scripts_basic FORCE
-	$(Q)$(MAKE) $(build)=scripts build_docproc
-	$(Q)$(MAKE) $(build)=doc/DocBook $@
+DOC_TARGETS := xmldocs latexdocs pdfdocs htmldocs epubdocs cleandocs \
+	       linkcheckdocs dochelp refcheckdocs
+PHONY += $(DOC_TARGETS)
+$(DOC_TARGETS): scripts_basic FORCE
+	$(Q)$(MAKE) $(build)=Documentation $@
 
 endif #ifeq ($(config-targets),1)
 endif #ifeq ($(mixed-targets),1)