[oe] Fix relocation of our SDK toolchains

[Tom Rini]

Hey all.  For some time, myself and others have wished that the
toolchain we install with our SDK target (meta-toolchain*) could be
relocatible.  I've finally spent some time digging into this and I've
come up with the following series of patches.  We had two problems.  The
first is that things like 'as', 'ld', 'cpp', etc that the toolchain
invokes were absolute links rather than relative links within the SDK.
The second issue is that we were using --with-gxx-include-dir in
configuring gcc when really we don't want that, we want to put the C++
includes where GCC is going to look for them already, not in an absolute
path.

I've tested this series of patches with gcc-4.1.2 / 4.2.4 and 4.3.3
cross-sdk targets on Linux and 4.2.4 on Windows.  For Linux I've
compiled qtopia-core-4.3.5 as a stand-in for a large complex app.

At this point the only thing left is to make the environment-setup setup
based on where it's invoked from.

-- 
Tom Rini