net: tftp: Fix too small block size

net: tftp: Fix too small block size

Commit b618b3707633 ("net: Convert CONFIG_TFTP_BLOCKSIZE to Kconfig")
accidentally set the default *option* TFTP block size to 512 bytes, even
though the comment in the code says that this is a terrible choice. Most
boards didn't define the symbol before, so they got the default block size
of 1468 bytes before, but now use 512 bytes, which is also the fallback.
This leads to both abysmal performance and a lot of hashes printed
on the screen (one character for every 5K), which is both annoying and
slow over serial links.

Set the default block size in Kconfig back to the value it had before.

This improves TFTP performance from 2.8 MB/s to 6.9 MB/s on a Pine64.

Fixes: b618b3707633 ("net: Convert CONFIG_TFTP_BLOCKSIZE to Kconfig")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>