scripts/functions: add target_endian_le and target_endian_be

We currently define target_endian_el and target_endian_eb to be the
tuple extension depending on endianness, defined to be respectively
'el' or 'eb' according to the endianness.

Some architecture do not use 'el' or 'eb', but use 'le' or 'be'.

Provide that as well, as two new variables: target_endian_le and
target_endian_be.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Cody P Schafer <dev@codyps.com>

# 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"