scripts/xldd: use user's sed and grep

xldd uses sed and grep as detected by ./configure. This works well if is
used on the machine that build the toolchain.

But if the user moves the toolchain to another machine where sed and grep
are not in the same directory (eg. /bin/sed vs. /usr/bin/sed), then xldd
will stop functionning.

Fix that by using ${SED} and ${GREP} if they are set in the environment.

Reported-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.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"