Line #52 of /usr/share/udhcpc/default.script makes use of a postfix increment operator, which is not defined by POSIX and not supported by shells such as dash.
$ sed -n 52p /usr/share/udhcpc/default.script
route add default gw $i dev $interface metric $((metric++))
Considered in isolation, the matter could be resolved by writing it as follows.
route add default gw $i dev $interface metric $metric
: $(( metric += 1 ))
As an aside, the preceding seven lines look suspicious in so far as `ip route` is potentially invoked without testing whether ip(8) is available - which is inconsistent with the rest of the script - before going on to execute a seemingly equivalent route(8) command in that event.