kconfig: install compiled frontends

The kconfig frontends are currently instaleld as source files. This is
a remnant of the early times, when I wanted a single installation of
crosstool-NG to be shared across multiple machines, potentially of
different architectures.

This does not really make sense, and it's been a long time since it
was las tpossible in practice.

So, just build the kconfig frontends at make-time, and install them
as we do for all other crosstool-NG dependent files.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Bare metal config options

 ## select BARE_METAL

 ##

 ## help 'Bare metal' refers to those programs that run without any kernel.

 ## help

 ## help You probably want to say 'y' here if you plan to use your compiler

 ## help to build bootloaders. It is not yet suitable to build Linux kernels,

 ## help though, because the APCI stuff relies on the target C library headers

 ## help being available?!?!...