flash node: improve flashing performance

flash node: improve flashing performance

Decreased the flashing time from 16m to 6m40s.

Conducted a dd test locally on the BMC to determine the correct block
size for writing to a raspberry CM4.
Times measured using a 206Mb file:

512B     1m50.22s
512K     43.0s
1M       43.03s
4M       42.96s
8M       43.19s
16M      43.00s
32M      45.95s

Concluded 4M is the optimal blocksize.

Ran some tests writing `2023-05-03-raspios-bullseye-armhf-lite.img`
* using dd 4M                   => 6m23.86s
* using tpi with -l flag        => 6m22s
* using tpi over local network  => 6m40s