platform: introduce hardware platform abstraction

These objects describes the hardware an image runs on. Including
 - architecture
 - bootloader
 - required firmware

Use the platform abstraction to move firmware packages out of the package set
definitions.

internal/platform/aarch64.go

@@ -0,0 +1,29 @@
+package platform
+
+type Aarch64 struct {
+	BasePlatform
+	UEFIVendor string
+}
+
+func (p *Aarch64) GetArch() Arch {
+	return ARCH_AARCH64
+}
+
+func (p *Aarch64) GetUEFIVendor() string {
+	return p.UEFIVendor
+}
+
+func (p *Aarch64) GetPackages() []string {
+	packages := p.BasePlatform.FirmwarePackages
+
+	if p.UEFIVendor != "" {
+		packages = append(packages,
+			"dracut-config-generic",
+			"efibootmgr",
+			"grub2-efi-aa64",
+			"grub2-tools",
+			"shim-aa64")
+	}
+
+	return packages
+}