Apply a timeout to kernel config in LFS

If the kernel-config file is not up to date (missing options or
whatever), the kernel "make" will hang, waiting for input. So
run "make oldconfig" explicitly, with a timeout.

Fixes: #1736

LFS/lfs.xsl

@@ -415,6 +415,8 @@ echo -e "\n\nTotalseconds: $SECONDS\n"
         <xsl:text>make mrproper&#xA;</xsl:text>
         <xsl:if test="ancestor::sect1[@id='ch-bootable-kernel']">
           <xsl:text>cp -v ../kernel-config .config&#xA;</xsl:text>
+          <xsl:text>timeout 60 make oldconfig ||\&#xA;</xsl:text>
+          <xsl:text>{ echo kernel config is not up to date; exit 124; }&#xA;</xsl:text>
         </xsl:if>
       </xsl:when>
 <!-- test instructions -->