Do not require .config to exist in kernel sources.

configure.ac

@@ -107,11 +107,6 @@ AC_ARG_WITH([linux-dir],
 
 AC_MSG_CHECKING([for Linux kernel sources])
 
-if test \! -r ${sourcedir}/.config; then
-    echo
-    AC_MSG_ERROR([No configured Linux kernel sources in $sourcedir])
-fi
-
 # Try to get kernel release string
 if test -r ${sourcedir}/include/config/kernel.release; then
     kernelrelease=`cat $sourcedir/include/config/kernel.release`
@@ -127,7 +122,7 @@ fi
 
 if test -z "$kernelrelease"; then
     echo
-    AC_MSG_ERROR([Failed to extract Linux kernel version!])
+    AC_MSG_ERROR([Failed to extract Linux kernel version! Does ${sourcedir} contain prepared kernel sources?])
 fi
 
 if test ${kernelrelease%%.*} -gt 2; then