UART Application Kit quick note:

Install Digi Embedded Linux 5.2 (RevB or RevA)

Install patches via PackageManager:

Package_Manager_EL
revision_patch
del-5_2-revB_upgrade
del-5_2_revB_revision_fix
del-5_2-hf1 <= this one includes UART AppKit support as external module

create a new kernel/kernel modules/rootfs project e.g. for ConnectCore 9M2443

In the kernel configuration (configure project) select:

Device Drivers -> Character devices -> Serial drivers -> 8250/16550 and compatible
  as "loadable module" which is a "." in the graphical configuration tool (not a check) SERIAL_8250 = m

also enable:

External Modules Configuration -> Kernel external modules -> UART application kit (DEL_KMOD_UARTAPPKIT)

Build and install kernel and rootfs and deploy it to the module.

During runtime when UART appkit is connected load the drivers (can also be done via
an .sh script from /etc/init.d/) with:

# modprobe 8250_uak
# modprobe uart_appkit

See also Help->Contents->Digi ESP for Embedded Linux-> 19.2 UART Application Kit

This will create the devices on external UART:
/dev/ttySUAK0  PORTA
/dev/ttySUAK1  PORTB
/dev/ttySUAK2  PORTC
/dev/ttySUAK3  PORTD