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>

# Dummy config file to not use a C library *at all*

## depends on BARE_METAL

##

## help Do not use a C library.

## help

## help This is usefull if your project is self-contained, does not rely on

## help an external C library, and provides all the necessary bits.

## help

## help Most probably usefull to bootloaders, as they generally don't depend

## help on the C library.

## help

## help If unsure: do *not* choose that, and use another option in the choice.