scripts: add support for building manuals

Add support for building the HTML and PDF manuals for the major
components. Implement for binutils, GCC, GDB, and GLIBC.

Always build all manuals and install a subset. Be explicit about the
subset to reduce the clutter and to avoid getting copies of common
manuals like bfd from all of the sourceware based components. Downside of
being explicit is that you need to update it when a new component
comes along.

Build the manuals as part of the last GCC build, namely 'cc' for glibc
based ones and cc_core_pass_2 for baremetal.

An example of the output is at:
http://people.linaro.org/~michaelh/incoming/crosstool-NG/

Signed-off-by: Michael Hope <michael.hope@linaro.org>
[yann.morin.1998@anciens.enib.fr: depends on ! remove docs; gold manual install]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Options specific to crosstool-NG acting as a backend

config IS_A_BACKEND

    string

    option env="CT_IS_A_BACKEND"

config BACKEND

    bool

    default y if IS_A_BACKEND = "y" || IS_A_BACKEND = "Y"

config BACKEND_ARCH

    string

    option env="CT_BACKEND_ARCH"

config BACKEND_KERNEL

    string

    option env="CT_BACKEND_KERNEL"

config BACKEND_LIBC

    string

    option env="CT_BACKEND_LIBC"