image: Fix instability of do_image_* checksums

The do_image_* tasks contained the expanded version of DATETIME. Due
to the expansion, we couldn't exclude the value from the task checksum
which meant the task would rerun.

We fix this by deleting the DATETIME value during expansion so we don't
expand any references to at that time. This means the task's hash can be
stable rather than having hardcoded date/time values. It will get expanded
at execution time.

This also fixes errors shown by -S:

NOTE: Reparsing files to collect dependency data
Writing locked sigs to /home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/build/locked-sigs.inc
ERROR: Bitbake's cached basehash does not match the one we just generated (/home/pokybuild/yocto-autobuilder/yocto-worker/nightly-oe-selftest/build/meta/recipes-sato/images/core-image-sato.bb.do_image_tar)!
ERROR: The mismatched hashes were 77872792556367f1dde49a1425caf1a0 and 9bb0aca6286ab7dd22d3c69964beb665

(From OE-Core rev: ecbc1db7ed1f9848dee69507de8eb289b8ddeba0)

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

meta/classes/image.bbclass

@@ -366,6 +366,10 @@ python () {
         localdata.setVar('OVERRIDES', '%s:%s' % (realt, old_overrides))
         bb.data.update_data(localdata)
         localdata.setVar('type', realt)
+        # Delete DATETIME so we don't expand any references to it now
+        # This means the task's hash can be stable rather than having hardcoded
+        # date/time values. It will get expanded at execution time.
+        localdata.delVar('DATETIME')
 
         image_cmd = localdata.getVar("IMAGE_CMD", True)
         vardeps.add('IMAGE_CMD_' + realt)
@@ -394,6 +398,7 @@ python () {
         d.setVarFlag('do_image_%s' % t, 'postfuncs', 'create_symlinks')
         d.setVarFlag('do_image_%s' % t, 'subimages', subimages)
         d.appendVarFlag('do_image_%s' % t, 'vardeps', ' '.join(vardeps))
+        d.appendVarFlag('do_image_%s' % t, 'vardepsexclude', 'DATETIME')
 
         after = 'do_image'
         for dep in typedeps[t]: