Create all the keyboard V3 layouts (SVG)

See merge request reform/reform!46

release version 2023-07-03

See merge request reform/reform!43

allow setting preprocessor variables from the outside

See merge request reform/reform!42

 - one no longer has to edit files to build different variants of the
   firmwares
 - set SOURCE_DATE_EPOCH to the date of the latest git commit for
   reproducibility
 - allow setting KBD_VARIANT_* from the outside
 - KBD_VARIANT_3_US and KBD_VARIANT_2_US set KBD_VARIANT_QWERTY_US
 - rename KBD_VARIANT_STANDALONE to KBD_MODE_STANDALONE
 - allow setting KBD_VARIANT_STANDALONE from the outside
 - build KBD_VARIANT_2 and KBD_VARIANT_3 with and without KBD_MODE_STANDALONE
   in the gitlab CI
 - allow setting KBD_FW_VERSION from the outside
 - set KBD_FW_REV to R2 for all variants and to R3 for KBD_VARIANT_3
 - rename REFORM_MBREV_R3 to REFORM_MBREV_20_R3
 - rename REFORM_MBREV_25_R1 to REFORM_MBREV_25_R2
 - allow setting REFORM_MOTHERBOARD_REV from the outside
 - pick FW_STRING2 depending on REFORM_MOTHERBOARD_REV
 - allow setting FW_STRING3 from the outside
 - build firmware on CI with git tag name in the version or commit hash

add .gitlab-ci.yml

See merge request reform/reform!41

Merge MB2.5 WIP and Trackball2 fw fixes

See merge request !40

flash.sh: make script interactive (no need for additional keyboard anymore)

See merge request !39

Merge imx8mplus adapter, keyboard3-pcb and trackball2-fw WIPs

See merge request reform/reform!38