Configure the ARM compiler to default to Cortex-A8

Ideally, we would only need to set this in build/jam/board/*, but the
flags set there are not passed to the build of packages. The default is
using some early ARM variant, for which gcc lacks some more atomic
operations and emits calls to helper functions we don't implement.

Setting the default architecture avoids this, as all packages will now
be built to target the Cortex-A8.

Also set the proper VFP version in BeagleBoard config file.

Note this breaks the Verdex and Pi builds, but ARMv7 is what we should
focus on for now. We can try to make older archs work after finishing
the m68k port.