deviceinfo: add initfs_extra_compression

For configuring the archive compression parameters for the
initramfs-extra archive.

doc/mkinitfs.1.scd

@@ -33,6 +33,7 @@ mkinitfs reads deviceinfo values from */etc/deviceinfo*. The following variables
 are *required* by mkinitfs:
 
 	- deviceinfo_initfs_compression
+	- deviceinfo_initfs_extra_compression
 	- deviceinfo_mesa_driver
 	- deviceinfo_modules_initfs
 	- deviceinfo_uboot_boardname
@@ -40,6 +41,10 @@ are *required* by mkinitfs:
 It is a design goal to keep the number of required variables from deviceinfo to
 a bare minimum, and to require only variables that don't hold lists of things.
 
+*NOTE*: When deviceinfo_initfs_extra_compression is set, make sure that the
+necessary tools to extract the configured archive format are in the initramfs
+archive.
+
 # DIRECTORIES
 
 The following directories are used by mkinitfs to generate the initramfs and

pkgs/deviceinfo/deviceinfo.go

@@ -14,10 +14,11 @@ import (
 )
 
 type DeviceInfo struct {
-	InitfsCompression string
+	InitfsCompression      string
-	MesaDriver        string
+	InitfsExtraCompression string
-	ModulesInitfs     string
+	MesaDriver             string
-	UbootBoardname    string
+	ModulesInitfs          string
+	UbootBoardname         string
 }
 
 func ReadDeviceinfo(file string) (DeviceInfo, error) {

pkgs/deviceinfo/deviceinfo_test.go

@@ -21,6 +21,7 @@ func TestNameToField(t *testing.T) {
 		{"deviceinfo_modules_initfs", "ModulesInitfs"},
 		{"modules_initfs", "ModulesInitfs"},
 		{"deviceinfo_modules_initfs___", "ModulesInitfs"},
+		{"deviceinfo_initfs_extra_compression", "InitfsExtraCompression"},
 	}
 
 	for _, table := range tables {