This adds NOR-style masking emulation to emubd when erase_value is set
to -2:
erase => 0xff
prog 0xf0 => 0xf0
prog 0xcc => 0xc0
We do _not_ rely on this property in littlefs, and so this feature will
probably go unused in our tests, but it's useful for running other
filesystems (SPIFFS) on top of emubd.
It may be a bit of a scope violation to merge this into littlefs's core
repo, but it's useful to centralize emubd's features somewhere...
Christopher Haster
92af5de3ca