Free serial port

By default Raspbian configures the Raspberry Pi serial port to provide boot-up information and allows login via a connected device.

Edit /etc/inittab:

sudo nano /etc/inittab

Add a # character at the beginning of line:

#T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

Exit and save.

Disable boot messages

Edit /boot/cmdline.txt:

sudo nano /boot/cmdline.txt

Change the line:

dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

to:

dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

Exit, save and reboot.