samples/task_profiler: fix crash when starting profiler

When profiler is started from shell, platform information is
transmitted over uart by calling uart_poll_out(uart_dev)
directly in the shell fiber context which is executed when
the command is entered ('prof start')

Problem is that 'uart_dev' global variable is set to the UART
console device when prof_flush() first call is done.

Depending on when this call happens (application dependent,
especially in nano-apps which have to call prof_flush in
their main loop), 'prof start' may happen before 'uart_dev'
variable is actually set, which drives to shell fiber crash.

This patches moves sending platform information from shell
fiber to prof_flush function to avoid using non initialized
'uart_dev' variable.

Change-Id: Ie64f4418a745affa3fbd731db6bf9bb998b7c2f9
Signed-off-by: Fabrice Olivero <fabrice.olivero@intel.com>