Add description for flash_bootimg_librem5-devkit.lst

Describe what the flash_bootimg_librem5-devkit.lst script does in the
header comments.
Also add a comment to describe what the `mmc partconf` does.
4 jobs for add-script-description in 3 minutes and 36 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #52234
librem5
build:deb

00:01:25

passed #52233
librem5
build:docs

00:01:56

 
  Test
passed #52235
librem5
test:download:deb

00:01:18

passed #52236
librem5
test:download:pip

00:01:40