docs/help: rename defconfig/olddefconfig into

After commit 15f57d843296e244487ac0845a73247f9d6749b8, the defconfig target
has been renamed into savedefconfig, and olddefconfig into defconfig. However,
the help text and man page was not updated.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
[yann.morin.1998@free.fr: rename CONFIG -> DEFCONFIG]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <0c1dff89c9ee1672693f.1374042343@BEANTN0L019720>
Patchwork-Id: 259601

# 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"