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>

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