wic: Honor APPEND

APPEND is used to add additional parameters to kernel by features, such
as read-only-rootfs. So we should honor this variable when we compose
the kernel parameter in wic. I know we also can resolve this kind of
issue by using the .wks.in template introduced by commit 42e870c5ed
("image_types.bbclass: support template .wks.in files for wic"), but the
APPEND is needed by all the wks, it would seems pretty ridiculous to me
to change all the .wks to .wks.in and then foist the APPEND into them.
So the APPEND is definitely deserved to export to the wic directly.

[Yocto #12809]

(From OE-Core rev: 18981b8a457104391dfd94938c247eac04e4ed50)

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/wic/ksparser.py

@@ -246,6 +246,11 @@ class KickStart():
                     elif line.startswith('bootloader'):
                         if not self.bootloader:
                             self.bootloader = parsed
+                            # Concatenate the strings set in APPEND
+                            append_var = get_bitbake_var("APPEND")
+                            if append_var:
+                                self.bootloader.append = ' '.join(filter(None, \
+                                                         (self.bootloader.append, append_var)))
                         else:
                             err = "%s:%d: more than one bootloader specified" \
                                       % (confpath, lineno)