nixos-generate-config: add support for bcache

Add "bcache" to boot.initrd.availableKernelModules if a bcache device is
detected.

This fixes a problem I've had one too many times: I install NixOS and
forget to add "bcache", resulting in an unbootable machine (until fixed
with Live CD). Now NixOS will do it for me.

nixos/modules/installer/tools/nixos-generate-config.pl

@@ -258,6 +258,11 @@ foreach my $path (glob "/sys/class/{block,mmc_host}/*") {
     }
 }
 
+# Add bcache module, if needed.
+my @bcacheDevices = glob("/dev/bcache*");
+if (scalar @bcacheDevices > 0) {
+    push @initrdAvailableKernelModules, "bcache";
+}
 
 my $virt = `systemd-detect-virt`;
 chomp $virt;