#!/usr/bin/env bash
FOO="SYSCFG_lan_ifname='br1' SYSCFG_lan_ipaddr='10.0.0.1' SYSCFG_lan_netmask='255.255.255.0'"

while IFS= read -r -d ''; do
  [[ $REPLY = *=* ]] || {
    printf 'ERROR: Item %q is not in assignment form\n' "$REPLY" >&2
    continue
  }
  printf -v "foo_${REPLY%%=*}" '%s' "${REPLY#*=}"
done < <(xargs printf '%s\0' <<<"$FOO")

# show what we parsed for demonstration purposes

for var in "${!foo_@}"; do
  echo "$var has value: ${!var}"
done