filesystem: several fixes in the API file system for Cellular devices

[rubenmoral]

• Cellular devices do not have NP setting, so the default block size for them should be 1490.
• It's not possible to specify a write offset when uploading secure files, 0xFFFFFFFF (use current position) must be used.
• Cellular devices return a file size of 0 bytes when creating a new file.
• When uploading files bigger than the block size, the last offset must be used. Otherwise, the file is overwritten at offset 0.