Add two bash arrays, XTRA_RSYNC_{FROM,TO}, defined in
/etc/default/armbian-ramlog, and expand these in the two rsync
invocations in /usr/lib/armbian/armbian-ramlog.
The use of bash arrays simplifies the use of multiple arguments and also
allows for in-band commentary between array elements.
When expanding, safely test for these arrays to exist. This is, sadly,
a little verbose in bash, so, while here, split the rsync invocations to
use line continuations.
Fixes https://github.com/armbian/build/issues/2846
2606d17ec7