allow setting preprocessor variables from the outside

• 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_2 is an alias for 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_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