Fixed incorrect reliance on errno in emubd
Fixed incorrect reliance on errno in emubd

When running the tests, the emubd erase function relied on the value of
errno to not change over a possible call to unlink. Annoyingly, I've
only seen this cause problems on a couple of specific Travis instances
while self-hosting littlefs on top of littlefs-fuse.

geky/littlefs,geky/littlefs,geky/littlefs,littlefs-project/littlefs,littlefs-project/littlefs,geky/littlefs