tegra: Test on CONFIG_CMD_USB being enabled for distro bootcmd

Reuse the common logic to allow for BOOT_TARGET_DEVICES to list USB as a
possibility if we're building for a platform that will have USB but not
if we don't, so that we don't hit the link-time check for trying to have
USB boot on a non-USB system.

Acked-by: Tom Warren <twarren@nvidia.com>
Signed-off-by: Tom Rini <trini@konsulko.com>

include/configs/tegra-common-post.h

@@ -21,11 +21,18 @@
 #define CONFIG_SYS_NONCACHED_MEMORY	(1 << 20)	/* 1 MiB */
 
 #ifndef CONFIG_SPL_BUILD
+
+#if CONFIG_IS_ENABLED(CMD_USB)
+# define BOOT_TARGET_USB(func) func(USB, usb, 0)
+#else
+# define BOOT_TARGET_USB(func)
+#endif
+
 #ifndef BOOT_TARGET_DEVICES
 #define BOOT_TARGET_DEVICES(func) \
 	func(MMC, mmc, 1) \
 	func(MMC, mmc, 0) \
-	func(USB, usb, 0) \
+	BOOT_TARGET_USB(func) \
 	func(PXE, pxe, na) \
 	func(DHCP, dhcp, na)
 #endif