build: work around bash issue

build: work around bash issue

Older bash (observed with 3.2.57(2)) fails to honor "set -e" for certain
built-in commands ("while" here), despite the command's status correctly
being non-zero. The subsequent objcopy invocation now being separated by
a semicolon results in no failure. Insert an explicit "exit" (replacing
; by && ought to be another possible workaround).

Fixes: e321576f4047 ("xen/build: start using if_changed")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>