kerneldoc: Implant DocBook from Linux kernel

kerneldoc: Implant DocBook from Linux kernel

Pull slightly modified version of Documentation/DocBook, the related perl
script scripts/kernel-doc and the scripts/docproc.c from Linux kernel and
implant it into U-Boot. This will allow smooth generation of kerneldoc
style documentation.

It was necessary to modify the DocBook/Makefile to work with U-Boot build
system. The changes were only minor though and involved replacing the kbuild
specific parts.

It was also necessary to replace use of variables like KERNEL_VERSION with
U_BOOT_VERSION, strings like Linux kernel with U-Boot Bootloader etc. so
the generated result actually matches.

Finally, it was necessary to adjust docproc.c, since the documentation in
U-Boot is located in doc/DocBook instead of Documentation/DocBook as is in
case of the Linux kernel.

Some parts of the DocBook Makefile are unused, but to allow easier sync with
Linux kernel, these parts are still left in. The targets enabled now are
"htmldocs" "pdfdocs" "psdocs" "xmldocs" and "cleandocs" to remove the results
of documentation build.

Linux scripts/docproc.c:
commit f0f3ca8d967462dafb815412b14ca3339b9817a6
Date:   Wed Jun 15 11:53:13 2011 +0200

Linux scripts/kernel-doc:
commit 1b40c1944db445c1de1c47ffd8cd426167f488e8
Date:   Sun Aug 12 10:46:15 2012 +0200

Linux Documentation/DocBook:
commit bb8187d35f820671d6dd76700d77a6b55f95e2c5
Date:   Thu May 17 19:06:13 2012 -0400

Signed-off-by: Marek Vasut <marex@denx.de>