Add new trace functions

- trace_printf_len: write specified amount of symbols from string to trace buffer
- trace_append: write format string to trace buffer

Signed-off-by: matsi <matsi@example.org>
Add define for IPU response string

Signed-off-by: matsi <matsi@example.org>
Do not enable mailbox interrupt until Linux on main CPU is booted

We can't enable interrupts in U-Boot. Let's wait until Linux is booted on CPU.
Linux remoteproc framework sends a ping to IPU via mailbox when its
initialization is completed. This ping is a good indicator for IPU that Linux
and its remoteproc/rpmsg subsystems are up an running.

Signed-off-by: matsi <matsi@example.org>
Cleanup

- add tags to gitignore
- clean-up trace logging

Signed-off-by: matsi <matsi@example.org>