doc: doxygen: use full path for USE_MDFFILE_AS_MAINPAGE

# (index.html). This can be useful if you have a project on for instance GitHub

# and want to reuse the introduction page also for the doxygen output.

USE_MDFILE_AS_MAINPAGE = mainpage.md

USE_MDFILE_AS_MAINPAGE = @ZEPHYR_BASE@/doc/_doxygen/mainpage.md

# The Fortran standard specifies that for fixed formatted Fortran code all

# characters from position 72 are to be considered as comment. A common