When the FS_END was adjusted to end on a full block (i.e. rounded down)
to avoid filesystem issues, but _FS_end was changed. The EEPROM library
used _FS_end to implicitly calculate the start of the EEPROM data, so
this means after the _FS_end fix, EEPROM data written with prior
releases would "disappear."
Avoid the issue by explicitly calculating the EEPROM start location in
the linker, using the same formula as prior release.
Fixes#6531
69f3e81fb9