build: allow relative paths of missing folders

readlink -f requires all but the last component of the path to exist.
readlink -m does not require components to exist.

This is useful for generating relative paths when the rule is going to
create the folder as well as the target in it.

Change-Id: Id2806acd9c749af7e022fae6a712d49427a0b5df
build: remove kconfig-frontends from source tree

Instead of distributing a snapshot of kconfig-frontends in the F4OS
source tree, make it an external dependency.

Since most Linux distributions do not provide a package for kconfig,
include a script to automatically download and install the tools.  This
means that the build flow is similar to before, except that new clones
must download kconfig-frontends, in addition to configuring and
building.

The build system will first look for the conf tools in the PATH.  If
they are not found there, it will fallback to the custom build.

Change-Id: I11d91456406745763b23f472b9c07644cd2ec6b0