kbuild: docbook: sync with Linux Kernel v3.15-rc1

kbuild: docbook: sync with Linux Kernel v3.15-rc1

This commit imports DocBook-related updates
from v3.14 to v3.15-rc1.

 - commit ec3fadd64b9940baa2a369bf978e8561837db3f5
    kbuild: docbook: use $(obj) and $(src) rather than specific path
 - commit 100da4c0150c97ce34d4d3b38bf2f5449b05ae4f
    kbuild: docbook: specify KERNELDOC dependency correctly
 - commit 6f89b9c1d6b29eaa600ac4a8ac1314b0d06f15e3
    kbuild: docbook: include cmd files more simply
 - commit ac6d662a95a6989d83b259ccf8ec01dd7903af73
    doc: Add "*.svg" to DocBook/.gitignore
 - commit 832919bfcf0cfd75767c68b0c61f7cf48be860a8
    DocBook: Make mandocs parallel-safe
 - commit c4d79a4799719f2b0cd354ee498aad605730c97e
    work around xmlto bug in htmldocs

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>