boot_serial: fix serial recovery mode with timeout

If 'BOOT_SERIAL_WAIT_FOR_DFU' is selected, the CPU shouldn't enter idle
state, waiting for interrupt from the console because we expect booting
if no mcumgr command is received within a configured timeout (with the
'CONFIG_BOOT_SERIAL_WAIT_FOR_DFU_TIMEOUT').

Without this fix, when using 'BOOT_SERIAL_WAIT_FOR_DFU' the boot process
hangs forever, waiting for input from console.

Fixes: e3822f81 ("boot_serial: zephyr: Add optional timeout to enter serial recovery")
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>