[poky] Cleanup task for bitbake.conf and Poky core cross and cross-canadian

[Richard Purdie]

Working on the canadian task, I'm reminded that some of the core
variables are a bit of a mess. There is a roadmap in mind to try and
clean this up which is something like the following:

Split target library generation away from the toolchain (gcc-runtime) [done]
Fix cross-canadian recipes to use TARGET_ARCH in PN [done]
Fix cross recipes to use TARGET_ARCH in PN

Once these two things are done it should allow us to start using
MULTIMACH_HOST_SYS instead of MULTIMACH_TARGET_SYS for the following
core variables in bitbake.conf: STAMP, WORKDIR, PKGDATA_DIR,
STAGING_KERNEL_DIR. 

The idea is if something is built to run on arch XXX then it should live
in the XXX workdir. Currently *-cross run on the HOST/BUILD architecture
but live in the target workdir.

Once we do this I think a lot of the mangling done in cross and
cross-canadian.bbclass should be able to be removed for variables like
STAGING_DIR_TARGET, STAGING_DIR_HOST, MULTIMACH_TARGET_SYS, PKGDATA_DIR,
PKGHIST_DIR, PKG_CONFIG_DIR, PKG_CONFIG_SYSROOT_DIR.

Also of note is that if we can obtain a sysroot per target machine and
default to that model, this means parts of bitbake.conf like:

# This should really be MULTIMACH_TARGET_SYS but that breaks "all" and machine 
# specific packages - hack around it for now.
STAGING_DIR_TARGET = "${STAGING_DIR}/${BASEPKG_TARGET_SYS}"

are no longer needed.

I thought I'd at least document these things since I just touched upon
them again with the cross-canadian changes.

Cheers,

Richard