configure: Detect kcov support and introduce CONFIG_KCOV

configure: Detect kcov support and introduce CONFIG_KCOV

kcov is kernel code coverage tracing tool. It requires kernel 4.4+
compiled with certain kernel options.

This patch checks if kcov header "sys/kcov.h" is present on build
machine, and stores the result in variable CONFIG_KCOV, meant to
be used in linux-user code related to the support for three ioctls
that were introduced at the same time as the mentioned header
(their definition was a part of the first version of that header).

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <1579214991-19602-11-git-send-email-aleksandar.markovic@rt-rk.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>