ffmpeg: enable or disable debug based on BR2_ENABLE_DEBUG

Rather than hard-code --disable-debug, selectively enable
or disable based on BR2_ENABLE_DEBUG.

(Similar to gst1-libav.)

[Thomas: do not make other changes such as reformatting the
--prefix=/usr line.]

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>