buildroot

jpeg: rework jpeg-turbo simd support handling

In preperation for adding more architectures to the jpeg-turbo simd
handling, introduce a hidden BR2_PACKAGE_JPEG_SIMDS_SUPPORT symbol and use
that to enable simd support and default to jpeg-turbo, instead of open
coding it in both places.

While we are at it, reword the help text to be more useful.

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