scripts/functions: use ${sed} instead of 'sed'

Helps build on BSD-like systems.

Reported-by: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
Signed-off-by: "Fabian Freyer" <fabian.freyer@physik.tu-berlin.de>
[yann.morin.1998@free.fr: split the original patch]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
configure: add option to specify grep

Allows users for which GNU grep is not the default grep (e.g. BSD
folks), or is in a weird location.

Reported-by: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
Signed-off-by: "Fabian Freyer" <fabian.freyer@physik.tu-berlin.de>
[yann.morin.1998@free.fr: split the original patch]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
scripts/crosstool-NG: use ${grep} instead of 'grep'

Helps building on BSD-like systems.

Reported-by: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
Signed-off-by: "Fabian Freyer" <fabian.freyer@physik.tu-berlin.de>
[yann.morin.1998@free.fr: split the original patch]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
Added documentation for --with-grep

Signed-off-by: "Fabian Freyer" <fabian.freyer@physik.tu-berlin.de>