v1.5.0

No major features added. The main change is the removal of mootif and
gtk backends and the build fix for better C23 standard compatibility.

• New -initramfs option to pass a separate initramfs file to the
  bootloader.
• --with-gtk and --with-x11 ./configure options (and backend
  code) are removed. xski binary is not built anymore.
• Build fix against -std=c23 toolchain.
• Pull in 'bootloader' code from linux-4.19.241.
• Switch from lex and yacc to flex and bison as dependencies.