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/boot/Makefile b/boot/Makefile
index a706742..124065a 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -10,8 +10,7 @@
 obj-$(CONFIG_CMD_BOOTZ) += bootm.o bootm_os.o
 obj-$(CONFIG_CMD_BOOTI) += bootm.o bootm_os.o
 
-obj-$(CONFIG_CMD_PXE) += pxe_utils.o
-obj-$(CONFIG_CMD_SYSBOOT) += pxe_utils.o
+obj-$(CONFIG_PXE_UTILS) += pxe_utils.o
 
 endif