uclibc: two-stage gcc simplifications

After switching to a two stage gcc solution, there is no longer a need
to do weird things in the uclibc build. We can greatly simplify
UCLIBC_CONFIGURE_CMDS to only do the configuration, and let the
existing UCLIBC_BUILD_CMDS do the build. Note that we have to build
the headers before starting the C library build, otherwise there is a
build failure (probably a uClibc bug).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>