package/fftw: break fftw-double into a dedicated package

package/fftw: break fftw-double into a dedicated package

fftw's library name depends on the precision option. Consequently,
it's possible to install multiple flavor on the same target.

This patch breaks fftw double precision into a new package and:

- makes BR2_PACKAGE_FFTW_PRECISION_DOUBLE select BR2_PACKAGE_FFTW_DOUBLE
  to keep compatibility with packages that use
  BR2_PACKAGE_FFTW_PRECISION_DOUBLE. This option will be removed in a
  follow-up commit;
- removes BR2_PACKAGE_FFTW_USE_SSE2 since it is only used for single
  and double precision. The corresponding options are directly handled in
  fftw-double package;
- makes fftw depend on fftw-double when this package is enabled;
- turns fftw itself into a generic-package instead of an
  autotools-package: it no longer builds anything and only allows to
  trigger the build of the appropriate fftw-* sub-packages, as a
  temporary compatibility mechanism.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
[Yann/Thomas:
 - Use FFTW_DOUBLE_DL_SUBDIR to avoid downloading fftw multiple times
 - Minor reformatting tweaks in fftw-double.mk
 - Drop from fftw.mk FFTW_CONF_ENV/FFTW_CONF_OPTS which are no
   longer used
 - Make fftw a generic package so that its dependencies are built
 - Do not deprecate BR2_PACKAGE_FFTW_PRECISION_DOUBLE and instead
   make it select BR2_PACKAGE_FFTW_DOUBLE, so that packages using
   BR2_PACKAGE_FFTW_PRECISION_QUAD continue to work.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>