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/platform.go

@@ -0,0 +1,58 @@
+package platform
+
+type Arch uint64
+
+const (
+	ARCH_AARCH64 Arch = iota
+	ARCH_PPC64LE
+	ARCH_S390X
+	ARCH_X86_64
+)
+
+func (a Arch) String() string {
+	switch a {
+	case ARCH_AARCH64:
+		return "aarch64"
+	case ARCH_PPC64LE:
+		return "ppc64le"
+	case ARCH_S390X:
+		return "s390x"
+	case ARCH_X86_64:
+		return "x86_64"
+	default:
+		panic("invalid architecture")
+	}
+}
+
+type Platform interface {
+	GetArch() Arch
+	GetBIOSPlatform() string
+	GetUEFIVendor() string
+	GetZiplSupport() bool
+	GetPackages() []string
+	GetBuildPackages() []string
+}
+
+type BasePlatform struct {
+	FirmwarePackages []string
+}
+
+func (p BasePlatform) GetBIOSPlatform() string {
+	return ""
+}
+
+func (p BasePlatform) GetUEFIVendor() string {
+	return ""
+}
+
+func (p BasePlatform) GetZiplSupport() bool {
+	return false
+}
+
+func (p BasePlatform) GetPackages() []string {
+	return p.FirmwarePackages
+}
+
+func (p BasePlatform) GetBuildPackages() []string {
+	return []string{}
+}