gltrace: Allow receiving commands of length > 4

Currently, gltrace offers very few trace collection options. As a
result, these options are encoded in a single integer. The trace
control task simply receives integers and interprets them as
commands.

This patch changes the control protocol to first receive the
command length followed by the actual command itself. This allows
for future flexibility to provide enable other commands.

Change-Id: Id5f56c80a025bbbe7613ab4457e092732e7d9dc9