buildroot

fdk-aac: refactor architecture dependency as a blind option

In preparation to the addition of a fdk-aac GStreamer plugin that will
have to select the fdk-aac package (and therefore inherits its
dependencies), this commits adds a BR2_PACKAGE_FDK_AAC_ARCH_SUPPORTS
blind option to avoid duplicating the architecture dependencies of
this package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>