msm: rtb: Separate initialization from enablement

The buffers for rtb are not set up immediately at boot time.
If a client tries to log into the buffer before these are set
up, rtb quietly exits. If enable is set on the command line,
rtb will try to log before the buffer is set up, causing crashes.
Fix this by creating a separate initialized variable to indicate
when the rtb buffer is set up.

Change-Id: Ia038347cd4b389cd0c82ac308c632404e000b17c
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>