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>

 # 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/