jenkins: reduce parallel, add verbose

V=s is openwrt, V=1 is here.
try and avoid weird problems with missing files like,
make[1]: *** No rule to make target '../../../include/libopencm3/stm32/g4/pwr.h', needed by 'rcc_common_all.o'.  Stop.

by turning down the paralellism a bit, it's a quad core arm, but it's
stil a little small machine.

tests/gadget-zero/Jenkinsfile

@@ -34,7 +34,7 @@ pipeline {
             steps {
                 sh '''
                     . .env3/bin/activate
-    		    make -j5 V=s
+                    make -j3 V=1
                     make -C tests/gadget-zero all V=s
                 '''
             }