archive.New: accept compression level (MR 25)

I went with a simpler implementation that uses Go compression packages
to do the work. The downside of this is that the compression Level is a
bit weird to set, since most libraries discourage setting the numeric
compression level directly.

This is configured by setting `deviceinfo_initfs_compression`, the value
it expects is a string in the form: `FORMAT[:LEVEL]`, where `[:LEVEL]`
is optional. Actually setting the variable at all is optional... if
nothing is specified, or it can't parse the format/level from the string
value, it defaults to using gzip with the "default" level for the
package (which tries to mirror gzip's default, or something).

The level can be one of `default`, `fast`, `best`.

To configure gzip with the fastest compression (so, bigger size): deviceinfo_initfs_compression="gzip:fast"`

To configure zstd with the most compression: `deviceinfo_initfs_compression="zstd:best"`

To configure zstd with default compression: `deviceinfo_initfs_compression="zstd"` (or `deviceinfo_initfs_compression="zstd:default"`)

In this case, `gzip:default` is assumed: deviceinfo_initfs_compression="bananas:mmmm"`

internal/archive/archive.go

@@ -45,14 +45,16 @@ type Archive struct {
 	cpioWriter      *cpio.Writer
 	buf             *bytes.Buffer
 	compress_format CompressFormat
+	compress_level  CompressLevel
 }
 
-func New(format CompressFormat) (*Archive, error) {
+func New(format CompressFormat, level CompressLevel) (*Archive, error) {
 	buf := new(bytes.Buffer)
 	archive := &Archive{
 		cpioWriter:      cpio.NewWriter(buf),
 		buf:             buf,
 		compress_format: format,
+		compress_level:  level,
 	}
 
 	return archive, nil