Add scripts/reduce-config.pl

The script can be used to generate a minimalistic kernel config file.
All lines, which don't make a difference to the resulting
configuration, are removed.

To use:

  - cd into a kernel build directory
  - copy the input configuration to config.in
  - run this script

The minimalistic config file can the be found in config.reduced.

The script works by removing lines from the config file one by one,
runnig  "make olddefconfig" and comparing the results. If the removal
of a input line didn't make a difference, it is dropped permanently
from the output.

scripts/reduce-config.pl

@@ -0,0 +1,86 @@
+#! /usr/bin/perl
+use strict;
+use warnings;
+
+our $CONFIG_IN='./config.in';
+our $CONFIG_OUT='./config.reduced';
+
+my @lines;
+my @disa;
+
+sub read_config {
+	my ($fn)=@_;
+	open my $in,'<',$CONFIG_IN or die "$CONFIG_IN: $!\n";
+	@lines=(<$in>);
+}
+
+sub write_config {
+	my ($fn)=@_;
+	open my $out,'>',$fn or die "$fn: $!\n";
+	for (my $i=0;$i<@lines;$i++) {
+		print $out $lines[$i] unless $disa[$i];
+	}
+	close $out;
+}
+
+sub build {
+	write_config('.config');
+	system 'make olddefconfig>/dev/null';
+	if ($?) {
+		warn "make failed: $?\n";
+		return 0;
+	}
+	return 1;
+}
+
+sub try {
+	write_config('./config');
+	build() or return 0;
+	system 'cmp','-s','.config','.config.target.tmp';
+	if ($?==0) {
+		return 1;
+	} else {
+		return 0;
+	}
+}
+
+read_config($CONFIG_IN);
+build() or die "failed to build with original config\n";
+system 'cp','.config','.config.target.tmp' and exit 1;
+
+my $p=0;
+my $progress=0;
+
+while (1) {
+	if ($disa[$p]) {
+		;
+	} else {
+		$disa[$p]=1;
+		if (try) {
+			$progress=1;
+			warn "disable $lines[$p]";
+		} else {
+			$disa[$p]=0;
+			warn "keep    $lines[$p]";
+		}
+	}
+	$p++;
+	if ($p>=@lines) {
+		if ($progress) {
+			warn "starting over\n";
+			$p=0;
+			$progress=0;
+		}
+		else {
+			warn "no progress. finished\n";
+			last;
+		}
+	}
+}
+
+# generate a valid .config
+try() or die "final compare failed\n";
+
+# and save the result
+write_config($CONFIG_OUT);
+