Makefile: Add flashing mode configuration

Signed-off-by: Angus Ainslie <angus@akkea.ca>
2 jobs for debian/master in 1 minute and 18 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Package
passed #338158
librem5
package:deb-debian-buster

00:00:48

 
  Test Package
passed #338159
librem5
lintian-debian-buster-package

00:00:29