toolchain: print actual version of kernel headers when checking

toolchain: print actual version of kernel headers when checking

Since we introduced the _AT_LEAST_XXX for the kernel headers, people
using pre-built custom toolchain now have to specify the version of
the kernel headers their custom toolchain uses.

So, when we detect that there is a mismatch between the selection in
the menuconfig, and the actual version of the headers, we currently
only bail out with a terse message "Incorrect selection of kernel
headers".

This could be confusing some, and getting the version of the headers
used by the toolchain is not trivial (well, it's very easy, but not
trivial.)

This patch changes the way we report the error by moving the message
into the test-code, and by printing the expected and actual versions
of the kernel headers.

BUT! To get this pretty error message, we need to run the
test-program, so we can not use the cross-toolchain, we have to use
the native one.

BUT! The native one has its own linux/version.h header, so we can not
simply include it.

So, we ask the cross-compiler where its default sysroot is, and use
that to then force-feed the cross linux/version.h to the native
toolchain.

[Thomas: augment commit log with a message provided by Yann, fix
coding style to not have spaces after opening parenthesis and before
closing parenthesis, reformatted the message "Incorrect selection..."
to make it fit on one line.]

Reported-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>