manifest: update to latest sof tree

Update to a more recent SOF commit:
87de994f49b37e385e748e47b769140df43451cf

This commit also has the SOF app integrated, so we do not need the sof
sample in the tree anymore and able to run the one from SOF directly.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>