package/Makefile.in: set variables for legacy f77 checks

For fortran detection, some projects check for fortran availability
using the FC/FCFLAGS variables, and others for the legacy F77/FFLAGS
ones.
So, make sure the legacy fortran F77 and FFLAGS variables are set in
TARGET_CONFIGURE_OPTS.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>