dhcp_%.bbappend: fix replaces original key warning
Use ${PN} rather than dhcp to keep align with oe-core to fix:
WARNING: oe-core/meta/recipes-connectivity/dhcp/dhcp_4.3.4.bb: Variable key SYSTEMD_AUTO_ENABLE_${PN}-client (enable) replaces original key SYSTEMD_AUTO_ENABLE_dhcp-client (disable).
WARNING: oe-core/meta/recipes-connectivity/dhcp/dhcp_4.3.4.bb: Variable key SYSTEMD_SERVICE_${PN}-client (dhclient.service) replaces original key SYSTEMD_SERVICE_dhcp-client (dhclient.service).
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>