UBUNTU: [debian] Build extras package only for specific arches

UBUNTU: [debian] Build extras package only for specific arches

An unwanted side effect of renaming arm omap4 to generic is that
the default rule is to create an extras package for flavours named 'generic'.
Furthermore, We stupidly tied the extras package logic to the flavour name 'generic'.

Defeat this side effect by specifying which architectures get an extras package
split, e.g., x86_64 and i386 in the arch specific make file.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>