replace the OUTPUT=IPL with equivalent OUTPUT=RAMDISK

The initial PR with s390 support in ReaR introduced an s390-only OUTPUT=IPL
undocumented option.  However, the OUTPUT=IPL option is completely redundant
because it does the exact same thing as the already existing and documented
OUTPUT=RAMDISK option.

This commit removes the whole IPL directory sub-tree and introduces a fallback
that replaces OUTPUT=IPL with OUTPUT=RAMDISK during the prep phase with
a deprecation warning to still be backwards compatible with existing local.conf
files.

Related: rear#3149 (comment)

usr/share/rear/prep/Linux-s390/034_check_config.sh

@@ -0,0 +1,8 @@
+if [ "$OUTPUT" = "IPL" ]; then
+    LogPrintError "Warning: OUTPUT=IPL is deprecated. Use OUTPUT=RAMDISK instead."
+    OUTPUT=RAMDISK
+fi
+
+if [ "$OUTPUT" != "RAMDISK" ] ; then
+   Error "Currently, only OUTPUT=RAMDISK is supported on s390/s390x"
+fi