Cleanup TARGET_CONFIGURE_OPTS

The definition of CC, LD, GCC, CPP, CXX and FC shouldn't contain the
CFLAGS/LDFLAGS/CXXFLAGS, those should be passed through the
appropriate variables.

However, the --sysroot option is a particular case here: it needs to
be part of the CC/LD/GCC/etc. definitions otherwise libtool strips it
from the CFLAGS/LDFLAGS.

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