package/pkg-python: use _CONFIGURE_OPTS in the build environment

When building host or target python packages, we need to ensure that
the build environment utilize {HOST|TARGET}_CONFIGURE_OPTS. This
ensures that the correct linker and compiler environment variables are
set to compile utilizing either the host or target folders.

It was discovered that when compiling a host-python package, it was
using linking against the build machines library folder instead of the
host folder because LDFLAGS was not properly set and was improperly
detecting whether or not a shared or static library was present in the
host folder.

CC: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Ryan Barnett <ryan.barnett@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>