Rework sysroot option handling

The external toolchain and internal toolchain cases both need to use
the --sysroot option, and they have almost identical
LDFLAGS/CFLAGS/CXXFLAGS definition, so we can factorize these
definitions.

Moreover, the --isysroot option is implied by --sysroot so there's no
need to specify both.

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