package/micropython: require python3 on the host

There are some scripts used when building micropython that require
python3 on the build host. Use BR2_PYTHON3_HOST_DEPENDENCY so this can
be either be satisfied by installing it on the build host or by building
the host-python3 package.

Fixes:
- http://autobuild.buildroot.net/results/b85b2214576030026a8e04d142a77a2648379417/

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/micropython/micropython.mk

@@ -8,7 +8,7 @@ MICROPYTHON_VERSION = 1.11
 MICROPYTHON_SITE = $(call github,micropython,micropython,v$(MICROPYTHON_VERSION))
 MICROPYTHON_LICENSE = MIT
 MICROPYTHON_LICENSE_FILES = LICENSE
-MICROPYTHON_DEPENDENCIES = host-pkgconf libffi
+MICROPYTHON_DEPENDENCIES = host-pkgconf libffi $(BR2_PYTHON3_HOST_DEPENDENCY)
 
 # Set GIT_DIR so package won't use buildroot's version number
 MICROPYTHON_MAKE_ENV = \