imx-usb-loader: propagate libusb dependency

Avoid select of libusb when the toolchain does not support threads.

Fixes:
http://autobuild.buildroot.net/results/a21/a212ee3e5ec174f4e6e7d582f777325dac035bfd/

Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/imx-usb-loader/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_IMX_USB_LOADER
 	bool "imx-usb-loader"
+	depends on BR2_TOOLCHAIN_HAS_THREADS # libusb
 	select BR2_PACKAGE_LIBUSB
 	help
 	  This package contains tools to download and execute code
@@ -7,3 +8,6 @@ config BR2_PACKAGE_IMX_USB_LOADER
 	  Serial Download Protocol.
 
 	  https://github.com/boundarydevices/imx_usb_loader
+
+comment "imx-usb-loader needs a toolchain w/ threads"
+	depends on !BR2_TOOLCHAIN_HAS_THREADS