image: Fix wic environment issues

The wic environment function needs to run after the rootfs size is
setup. We move this code to a specific task, and depend on that task
from the wic images and other places its needed.
This fixes:

======================================================================
FAIL: test_image_env (oeqa.selftest.wic.Wic)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/meta/lib/oeqa/utils/decorators.py", line 106, in wrapped_f
    return func(*args, **kwargs)
  File "/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/meta/lib/oeqa/selftest/wic.py", line 218, in test_image_env
    self.assertTrue(var in content, "%s is not in .env file" % var)
AssertionError: False is not true : ROOTFS_SIZE is not in .env file

(From OE-Core rev: 606f9e2d7d8d389c8d4f5c3090139d3bb780e09c)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/image.bbclass

@@ -242,8 +242,6 @@ fakeroot python do_image () {
     pre_process_cmds = d.getVar("IMAGE_PREPROCESS_COMMAND", True)
 
     execute_pre_post_process(d, pre_process_cmds)
-
-    write_wic_env(d)
 }
 do_image[dirs] = "${TOPDIR}"
 do_image[umask] = "022"
@@ -264,7 +262,7 @@ addtask do_image_complete after do_image before do_build
 # Write environment variables used by wic
 # to tmp/sysroots/<machine>/imgdata/<image>.env
 #
-def write_wic_env(d):
+python do_rootfs_wicenv () {
     wicvars = d.getVar('WICVARS', True)
     if not wicvars:
         return
@@ -278,6 +276,10 @@ def write_wic_env(d):
             value = d.getVar(var, True)
             if value:
                 envf.write('%s="%s"\n' % (var, value.strip()))
+}
+addtask do_rootfs_wicenv after do_rootfs before do_image_wic
+do_rootfs_wicenv[vardeps] += "${WICVARS}"
+do_rootfs_wicenv[prefuncs] = 'set_image_size'
 
 def setup_debugfs_variables(d):
     d.appendVar('IMAGE_ROOTFS', '-dbg')