alsa-utils: make fftw an optional dependency for alsabat

alsa-utils: make fftw an optional dependency for alsabat

The BR2_PACKAGE_ALSA_UTILS_BAT option currently selects
BR2_PACKAGE_FFTW. However, it turns out that fftw is not a mandatory
dependency to build alsabat, just an optional dependency.

This commit therefore turns this dependency into an optional
one. However, it is not just FFTW that is necessary, but the simple
precision version of FFTW, so the optional dependency takes this into
account.

We take this opportunity to extend the Config.in help text for
BR2_PACKAGE_ALSA_UTILS_BAT.

Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>