/bin/sh: 1: tools/mkconfig: not found
tools/Makefile.unix:247: recipe for target 'include/nuttx/config.h' failed
make: *** [include/nuttx/config.h] Error 127
Your problem is not that kconfig-frontends is not being built, but
rathter that tools/mkconfig is not being built. That should happen
automatically when you build NuttX because it is required by the
dependencies. If you use 'make V=1' you should have seen
tools/mkconfig being built. There should also be a tools/mkconfig
binary in your tools/directory.
If not, then the make rule is broken somehow in your case.
You can try building tools/mkconfig just as a test to see if that solves
the build problem: make -C tools -f Makefile.host
That is not a solution but is a good verification of the build failure.
