loader.efi: Expose efi_devpath_get_mac to get mac

This is a convenient way to test if a device path is a nic or not, so
expose it to the world.

Sponsored by:		Netflix

stand/efi/include/efilib.h

@@ -105,6 +105,7 @@ bool efi_devpath_is_prefix(EFI_DEVICE_PATH *, EFI_DEVICE_PATH *);
 CHAR16 *efi_devpath_name(EFI_DEVICE_PATH *);
 void efi_free_devpath_name(CHAR16 *);
 bool efi_devpath_same_disk(EFI_DEVICE_PATH *, EFI_DEVICE_PATH *);
+bool efi_devpath_get_mac(EFI_DEVICE_PATH *, uint8_t[6]);
 EFI_DEVICE_PATH *efi_devpath_to_media_path(EFI_DEVICE_PATH *);
 UINTN efi_devpath_length(EFI_DEVICE_PATH *);
 EFI_DEVICE_PATH *efi_name_to_devpath(const char *path);

stand/efi/libefi/efinet.c

@@ -267,13 +267,10 @@ efi_pxe_snapshot(EFI_HANDLE h, struct pxe_cache_entry *pce)
 
 /*
  * Walk a device path looking for the first MSG_MAC_ADDR_DP node and
- * copy its 6-byte Ethernet MAC into out.  Returns true on success.
- * Used to correlate a PXE Base Code Protocol handle back to an SNP
- * unit when the two protocols live on different handles (typical for
- * UEFI: PXE Base Code is installed on a child handle whose device
- * path ends in Ipv4()/Ipv6(), extending the SNP's MAC-terminated path).
+ * copy its 6-byte Ethernet MAC into out.  Returns true on success,
+ * e.g. false if dp has no NIC node at all.
  */
-static bool
+bool
 efi_devpath_get_mac(EFI_DEVICE_PATH *dp, uint8_t out[6])
 {
 	MAC_ADDR_DEVICE_PATH *mac;