Make rbstream pagesize smaller and device specific
Make rbstream pagesize smaller and device specific

The progress bar was taking 18 seconds between updates on a Commander II
when using a 128K pagesize. Since devices differ in their baud rates, it
makes sense to use smaller pages on slower devices. This change reduces
it to 32K on a Commander II and to 64K on EMC devices.

henrik242/libdivecomputer,henrik242/libdivecomputer