boot: allow bootmeth-distro without CONFIG_NET

Remove the dependency on CMD_PXE from BOOTMETH_DISTRO by introducing a
new hidden kconfig symbol to control whether pxe_utils is compiled,
allowing bootstd's distro method to be compiled without needing
networking support enabled.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Correct build errors when CMD_BOOTM is not enabled:
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 3625ff2..7d19706 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1825,7 +1825,7 @@
 
 config CMD_PXE
 	bool "pxe"
-	select MENU
+	select PXE_UTILS
 	help
 	  Boot image via network using PXE protocol
 
@@ -2007,7 +2007,7 @@
 
 config CMD_SYSBOOT
 	bool "sysboot"
-	select MENU
+	select PXE_UTILS
 	help
 	  Boot image via local extlinux.conf file