ash: main: Only set savestatus in exitcmd

ash: main: Only set savestatus in exitcmd

Upstream commit:

    Date: Sat, 19 May 2018 02:39:38 +0800
    main: Only set savestatus in exitcmd

    Currently exitcmd sets exitstatus and then savestatus if the latter
    was previously set.  In fact, as exitcmd always raises an exception
    and will either end up in the setjmp call in main() or exitshell(),
    where exitstatus is always replaced by savestatus if set, we only
    need to set savestatus.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>