toolchain/helpers: make sure we bail out when kernel headers check fails

In commit 6136765b ("toolchain:
generate check-headers program under $(BUILD_DIR)"), the
check_kernel_headers_version function was simplified to not check the
return value of the check-kernel-headers.sh script, assuming that
"make" does bail out on the first failing command.

However, check_kernel_headers_version when used in $(2)_CONFIGURE_CMDS
from pkg-toolchain-external.mk, is called in a sequence of commands,
where the return value of each command is not checked. Therefore, a
failure of check-kernel-headers.sh no longer aborts the build.

Since all other macros are using this principle of calling "exit 1",
we revert back to the same for check_kernel_headers_version, as it was
done prior to 6136765b.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Carlos Santos <unixmania@gmail.com>
Reviewed-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>