Disable firmware user mode helpers

We don't need them and it causes issues for postmarket os where the user
mode helper takes a long time to complete.

Closes: #31
Signed-off-by: Guido Günther <guido.gunther@puri.sm>
2 jobs for firmware in 40 minutes and 59 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #10233
librem5
build:aarch64:Image

00:40:59

passed #10234
librem5
build:aarch64:deb

00:27:45