Consolidate all the various console definitions to the common
conf/machine/include/rockchip-defaults.inc file and create
RK_CONSOLE_BAUD and RK_CONSOLE_DEVICE variables that can be
reused in the wks files.

The following variables were checked before and after this patch
to make sure they are sensible:
- SERIAL_CONSOLES
- RK_CONSOLE_DEVICE
- RK_CONSOLE_BAUD

A boot test was performed on the following boards to make sure
they all continue to boot to a cmdline:
- tinker-board
- rock-pi-e
- nanopi-m4-2gb
- rock64
- rock-pi-4b

Signed-off-by: Trevor Woerner <twoerner@...>
---
Changes from v1:
- In v1 I defined RK_CONSOLE_BAUD and RK_CONSOLE_DEVICE for each MACHINE
and then redefined SERIAL_CONSOLES to be the concatenation of these two
variables. Khem pointed out this is a bad approach because I'm redefining
an oe-core-defined variable that all BSPs expect.
- In v2 I set/consolidate SERIAL_CONSOLES for each MACHINE and then generate
RK_CONSOLE_BAUD and RK_CONSOLE_DEVICE based on the first-defined
<baud>;<device> pair found in SERIAL_CONSOLES; these generated variables are
then used in the wks files.