Replaced setVarFlag calls to appendVarFlag to allow
modification of prefuncs, postfuncs and subimages flags
in inherited image classes.
[YOCTO #11372]
(From OE-Core rev: 6690f3ab43c04fa7cff7215d4a5d8d639e41aed8)
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>