Symbol _filedata_start in netX51 and netJACK51 ELF files contains wrong address, causing wrong DataSize value in NXF CommonHeader, causing TagListEditor warning

This was already spotted earlier, but came up again from Varna with Test bug PRODEIS-160:

• Symptom is taglist editor and tagtool emitting a warning message as described in referenced ticket.
• Affected are firmwares which do INTRAM relocations. Currently, these are netX51 and netJACK51.
• Caused by wrong file offset being encoded into symbol _filedata_start during linking the binary.
• Fix is to move line "_filedata_start = . ;" within linker script to the actual start of the data in ELF file. Commonly, this is right behind the file header.