Reduce number of parallel gcc invocations from 4 to 2 per cpu

Some build hosts can run out of memory during compilation.
This should reduce memory demand without affecting build time.

Change-Id: I11bd2884a1f8885a8a332bbbf0a63324a3079c3b
Signed-off-by: Damjan Marion <damarion@cisco.com>

build-root/Makefile

@@ -670,7 +670,7 @@ linux_n_cpus = `grep '^processor' /proc/cpuinfo | wc -l`
 MAKE_PARALLEL_JOBS =				\
   -j $(shell					\
     if [ -f /proc/cpuinfo ] ; then		\
-      expr 4 '*' $(linux_n_cpus) ;		\
+      expr 2 '*' $(linux_n_cpus) ;		\
     else					\
       echo 1 ;					\
     fi)