Fixes in this commit:
- The wrong loader script was assigned for SAMD_GENERIC_D51X20, causing the
VFS block count to be wrong.
- Change the VFS block size from 1536 to 2048. With the setting of 1536,
writing more that 1536 bytes at once failed. This applies to
SAMD_GENERIC_D51X19 and SAMD_GENERIC_D51X20. No other SAMD51 board uses
the internal flash for the file system.
Signed-off-by: robert-hh <robert@hammelrath.com>