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>

 # MIPS specific config options

 ## select ARCH_SUPPORTS_32

 ## select ARCH_SUPPORTS_64 if EXPERIMENTAL

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORTS_BOTH_ENDIAN

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORTS_WITH_ARCH

 ## select ARCH_SUPPORTS_WITH_TUNE

 ## select ARCH_SUPPORTS_WITH_FLOAT

 ##

 ## help The MIPS architecture, as defined by:

 ## help     http://www.mips.com/