The Yocto Documentation, particularly the Complete Documentation Set and the Bitbake Users Manual provides a lot of information on these kind of questions.
Well that depends on what you want to do. If you want to spawn a login terminal on the USB serial connection then yes, you might want to set SERIAL_CONSOLES. If you want to make use of the serial gadget from within your own program you likely do not.
Note that the imx UART driver creates /dev/ttymxcX, not /dev/ttylpX.
The _append operator appends to the string already contained in a variable without adding whitspace. So
GREETING = "Hi"
GREETING_append = "there"
results in GREETING containing “Hithere”.
So it is likely that you want a leading whitespace with _append or a trailing whitespace with _prepend.
The _remove operator on the other hand searches the given word (or whitespace delimited words) in a variable and if they exist removes them. So no leading or trailing whitespace is needed for _remove.