[oe] RFC: Adding a new global MACHINE_ENDIAN variable

[Rod Whitby]

There exist machines (like the ixp4xx range of processors) which can be
run in either little-endian or big-endian mode, and both modes are
supported by OE.

I propose a new "MACHINE_ENDIAN" (naming courtesy of RP) variable to do
this selection (i.e. for machines that support both, the build can be
selected in local.conf).

NSLU2-Linux has previously had an "ARCH_BYTE_SEX" variable for this
purpose - I would move all instances of that to the new variable name.
I would also update Angstrom to use the new global variable name instead
of reusing the nslu2-linux-specific variable (which will be deprecated).

The supported values of the variable would be "be" and "le".

-- Rod