Update Coreboot Heads Librem 13v2, 13v4, 15v3, 15v4 image filenames

What?

The Coreboot Heads image filenames in the utility script don't match the current release (release 15) ROM filenames. The pre-compiled files have an underscore between "librem" and the model number.

Pre-compiled ROMs:

• librem_13v2: https://source.puri.sm/coreboot/releases/-/tree/master/librem_13v2
  • Filename: pureboot-librem_13v2-Release-15.rom.gz
• librem_13v4: https://source.puri.sm/coreboot/releases/-/tree/master/librem_13v4
  • Filename: pureboot-librem_13v4-Release-15.rom.gz
• librem_15v3: https://source.puri.sm/coreboot/releases/-/tree/master/librem_15v3
  • Filename: pureboot-librem_15v3-Release-15.rom.gz
• librem_15v4: https://source.puri.sm/coreboot/releases/-/tree/master/librem_15v4
  • Filename: pureboot-librem_15v4-Release-15.rom.gz

Why?

The utility script doesn't work when choosing the pre-compiled ROM option and receives a 404 when it attempts to use wget to download the .gz file.

How?

Update COREBOOT_HEADS_IMAGE_13v2, COREBOOT_HEADS_IMAGE_13v4, COREBOOT_HEADS_IMAGE_15v3, COREBOOT_HEADS_IMAGE_15v4 variables on these lines https://source.puri.sm/coreboot/utility/-/blob/master/coreboot_util.sh#L60-68.

COREBOOT_HEADS_IMAGE_13v2="pureboot-librem_13v2-${PUREBOOT_VERSION}.rom"
# ...
COREBOOT_HEADS_IMAGE_13v4="pureboot-librem_13v4-${PUREBOOT_VERSION}.rom"
# ...
COREBOOT_HEADS_IMAGE_15v3="pureboot-librem_15v3-${PUREBOOT_VERSION}.rom"
# ...
COREBOOT_HEADS_IMAGE_15v4="pureboot-librem_15v4-${PUREBOOT_VERSION}.rom"