arm-defaults.inc: Tweak DEFAULTTUNE with anonymous pything instead of overrides

This will avoid making entries for every machine on earch that is based
on armv7a and would like to use angstrom and its feeds, secondly it
makes it unified baseline for all armv7a machines and not divide it
with cortex-A8, cortex-A9 and so on
Above all brings sanity

Signed-off-by: Khem Raj <raj.khem@gmail.com>