loader: For EFI, if we don't have ConOut, try ConIn

Try ConIn if we don't have a ConOut variable. ConIn will contain HID
devices and/or serial devices. We currently just search for serial
devices and will use them instead of video with the current code. While
ConIn w/o ConOut is fairly common on laptops, is kinda rare on servers.
Some refinement may be needed in the future if servers come to
light. This is also minimal to allow possible integration into 13.3
release.

MFC After:		1 week
Sponsored by:		Netflix
Reviewed by:		dab, tsoome
Differential Revision:	https://reviews.freebsd.org/D43714

stand/efi/loader/main.c

@@ -722,7 +722,10 @@ setenv_int(const char *key, int val)
  * Parse ConOut (the list of consoles active) and see if we can find a
  * serial port and/or a video port. It would be nice to also walk the
  * ACPI name space to map the UID for the serial port to a port. The
- * latter is especially hard.
+ * latter is especially hard. Also check for ConIn as well. This will
+ * be enough to determine if we have serial, and if we don't, we default
+ * to video. If there's a dual-console situation with ConIn, this will
+ * currently fail.
  */
 int
 parse_uefi_con_out(void)
@@ -741,6 +744,8 @@ parse_uefi_con_out(void)
 	rv = efi_global_getenv("ConOut", buf, &sz);
 	if (rv != EFI_SUCCESS)
 		rv = efi_global_getenv("ConOutDev", buf, &sz);
+	if (rv != EFI_SUCCESS)
+		rv = efi_global_getenv("ConIn", buf, &sz);
 	if (rv != EFI_SUCCESS) {
 		/*
 		 * If we don't have any ConOut default to both. If we have GOP