progmem interface: Discern r/w and erase page sizes. Introduce new...

progmem interface: Discern r/w and erase page sizes.  Introduce new up_progmem_erasesize architecture API to be able to communicate the read/write page size independently from erase page size.  Implement up_progmem_erasesize for all architectures that already support progmem, assuming that for those architectures the sizes of both read/write and erase pages are identical.