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

package/fftw: break fftw-single 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 single precision into a new package and:

- makes BR2_PACKAGE_FFTW_PRECISION_SINGLE select
  BR2_PACKAGE_FFTW_SINGLE to keep compatibility with packages that use
  BR2_PACKAGE_FFTW_PRECISION_SINGLE. This option will be removed in a
  follow-up commit;

- removes the BR2_PACKAGE_FFTW_USE_SSE and BR2_PACKAGE_FFTW_USE_NEON
  otpions since they are only used for single precision. The
  corresponding CPU-capability options are directly handled in
  the fftw-single package;

- makes fftw depend on fftw-single when this package is enabled.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
[Yann/Thomas:
 - Force --disable-single in FFTW_CONF_OPTS, just for the sake of
   clarity (fftw is no longer going to build the single variant)
 - Use FFTW_SINGLE_DL_SUBDIR to avoid downloading fftw multiple times
 - Minor reformatting tweaks in fftw-single.mk
 - Do not deprecate BR2_PACKAGE_FFTW_PRECISION_SINGLE and instead make
   it select BR2_PACKAGE_FFTW_SINGLE, so that packages using
   BR2_PACKAGE_FFTW_PRECISION_SINGLE continue to work.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>