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>

 # Menu for the native GDB

 config GDB_NATIVE

     bool

     prompt "Native gdb"

     depends on ! BARE_METAL

     depends on ! BACKEND

     help

       Build and install a native gdb for the target, to run on the target.

 if GDB_NATIVE

 config GDB_NATIVE_STATIC

     bool

     prompt "Build a static native gdb"

     help

       In case you have trouble with dynamic loading of shared libraries,

       you will find that a static gdb comes in handy.

 endif # GDB_NATIVE