distro bootcmd: make net boot only optionally start USB

Currently, the distro boot commands always enumerate USB devices before
performing network operations. However, depending on the board and end-
user configuration, network devices may not be attached to USB, and so
enumerating USB may not be necessary. Enhance the scripts to make this
step optional, so that the user can decrease boot time if they don't
need USB.

This change is performed by moving the "usb start" invocation into a
standalone variable. If the user desires, they can replace that
variable's value with some no-op command such as "true" instead.

Booting from a USB storage device always needs to enumerate USB devices,
so this action is still hard-coded.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/config_distro_bootcmd.h

@@ -140,15 +140,16 @@
 #endif
 
 #ifdef CONFIG_CMD_USB
-#define BOOTENV_RUN_USB_INIT "usb start; "
+#define BOOTENV_RUN_NET_USB_START "run boot_net_usb_start; "
 #define BOOTENV_SHARED_USB \
+	"boot_net_usb_start=usb start\0" \
 	"usb_boot=" \
-		BOOTENV_RUN_USB_INIT \
+		"usb start; " \
 		BOOTENV_SHARED_BLKDEV_BODY(usb)
 #define BOOTENV_DEV_USB		BOOTENV_DEV_BLKDEV
 #define BOOTENV_DEV_NAME_USB	BOOTENV_DEV_NAME_BLKDEV
 #else
-#define BOOTENV_RUN_USB_INIT
+#define BOOTENV_RUN_NET_USB_START
 #define BOOTENV_SHARED_USB
 #define BOOTENV_DEV_USB \
 	BOOT_TARGET_DEVICES_references_USB_without_CONFIG_CMD_USB
@@ -159,7 +160,7 @@
 #if defined(CONFIG_CMD_DHCP)
 #define BOOTENV_DEV_DHCP(devtypeu, devtypel, instance) \
 	"bootcmd_dhcp=" \
-		BOOTENV_RUN_USB_INIT \
+		BOOTENV_RUN_NET_USB_START \
 		"if dhcp ${scriptaddr} ${boot_script_dhcp}; then " \
 			"source ${scriptaddr}; " \
 		"fi\0"
@@ -175,7 +176,7 @@
 #if defined(CONFIG_CMD_DHCP) && defined(CONFIG_CMD_PXE)
 #define BOOTENV_DEV_PXE(devtypeu, devtypel, instance) \
 	"bootcmd_pxe=" \
-		BOOTENV_RUN_USB_INIT \
+		BOOTENV_RUN_NET_USB_START \
 		"dhcp; " \
 		"if pxe get; then " \
 			"pxe boot; " \