Without this, when mkimage fails (for example because the boot partition
is too small), install.log shows an error like:
mkimage: Write only 16310208/17794920 bytes, probably no space left on the device
But this does not terminate the build, so the compile script shows no
error and an image is generated, but it is unbootable.
This runs the script with -e, so when mkimage fails, the script fails,
which makes update-initramfs fail, which makes the build fail.
