[oe] [rfc] change default for BUILD_OPTIMIZATION, remove spurious assignments

[Phil Blundell]

I'd like to propose changing the bitbake.conf default for
BUILD_OPTIMIZATION from "-O2" to "-O2 -g".  There seems no harm in
building debuggable binaries for host-side utilities and it makes life
somewhat easier when things go wrong.

Also, while investigating why assignments to this variable from
local.conf didn't do what I was expecting, I discovered that a whole lot
of files inside conf/, notably all of the sane-toolchain-*.inc files,
contain their own assignments to BUILD_OPTIMIZATION for no good reason
that I can see.  I would also propose to delete the assignments from
those files.  (There are a few other assignments in files that are
either DISTRO or MACHINE specific but I intend to leave those alone.)

p.