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/pxe_utils.c b/boot/pxe_utils.c
index defbe46..a364fa8 100644
--- a/boot/pxe_utils.c
+++ b/boot/pxe_utils.c
@@ -736,7 +736,8 @@
 	kernel_addr_r = genimg_get_kernel_addr(kernel_addr);
 	buf = map_sysmem(kernel_addr_r, 0);
 	/* Try bootm for legacy and FIT format image */
-	if (genimg_get_format(buf) != IMAGE_FORMAT_INVALID)
+	if (genimg_get_format(buf) != IMAGE_FORMAT_INVALID &&
+            IS_ENABLED(CONFIG_CMD_BOOTM))
 		do_bootm(ctx->cmdtp, 0, bootm_argc, bootm_argv);
 	/* Try booting an AArch64 Linux kernel image */
 	else if (IS_ENABLED(CONFIG_CMD_BOOTI))