deviceinfo: parse only required variables

This greatly reduces the chance accidentally adding dependencies to the
other (currently unused) variables later on. Getting away from depending
on deviceinfo has a lot of benefits, but mainly it helps offload
device-specific boot configuration to boot-deploy. Handling those
complexities in a shell script is often nicer.

Also, reducing the need to handle variables that contain lists means
that this app doesn't have to worry about how to merge/handle multiple
versions of those. That might be useful later if mkinitfs has to read
deviceinfo config from multiple deviceinfo files.

For example, trying to figure out how to merge these two things is...
ehhh...
        a_modules_initfs="abc bar banana bazz"
        b_modules_initfs="foo bar bazz bar2 guava"

pkgs/deviceinfo/deviceinfo.go

@@ -14,31 +14,10 @@ import (
 )
 
 type DeviceInfo struct {
-	AppendDtb                     string
-	Arch                          string
-	UbootBoardname                string
-	BootimgAppendSEAndroidEnforce string
-	BootimgBlobpack               string
-	BootimgDtbSecond              string
-	BootimgMtkMkimage             string
-	BootimgPxa                    string
-	BootimgQcdt                   string
-	Dtb                           string
-	FlashKernelOnUpdate           string
-	FlashOffsetBase               string
-	FlashOffsetKernel             string
-	FlashOffsetRamdisk            string
-	FlashOffsetSecond             string
-	FlashOffsetTags               string
-	FlashPagesize                 string
-	GenerateBootimg               string
-	GenerateLegacyUbootInitfs     string
-	InitfsCompression             string
-	KernelCmdline                 string
-	LegacyUbootLoadAddress        string
-	MesaDriver                    string
-	MkinitfsPostprocess           string
-	ModulesInitfs                 string
+	InitfsCompression string
+	MesaDriver        string
+	ModulesInitfs     string
+	UbootBoardname    string
 }
 
 func ReadDeviceinfo(file string) (DeviceInfo, error) {