[jenkins] Marvell ARMv7 armhf build improvements (sonic-net#201)

This commit provides the following improvements to the Marvell armhf builds
performed by Jenkins.

1. Improve build performance by attempting a build using SONIC_BUILD_JOBS=4. If
   that fails then try SONIC_BUILD_JOBS=2. If that fails then try
   SONIC_BUILD_JOBS=1. If that fails we are done.

2. Disable concurrent builds from the same branch. This strives to avoid a
   current armhf build limitation where multiple builds executed on the same
   build machine interfere with each other.

jenkins/marvell-armhf/buildimage-mrvl-armhf-all/Jenkinsfile

@@ -3,6 +3,7 @@ pipeline {
 
  options {
  buildDiscarder(logRotator(artifactDaysToKeepStr: '', artifactNumToKeepStr: '', daysToKeepStr: '', numToKeepStr: '30'))
+ disableConcurrentBuilds()
  }
 
  environment {
@@ -38,9 +39,9 @@ pipeline {
 git submodule foreach --recursive '[ -f .git ] && echo "gitdir: $(realpath --relative-to=. $(cut -d" " -f2 .git))" > .git'
 export DOCKER_DATA_ROOT_FOR_MULTIARCH=/data/march/docker
 make configure PLATFORM=marvell-armhf PLATFORM_ARCH=armhf
-make SONIC_CONFIG_BUILD_JOBS=1 INSTALL_DEBUG_TOOLS=y target/sonic-marvell-armhf.bin
+make SONIC_CONFIG_BUILD_JOBS=4 INSTALL_DEBUG_TOOLS=y target/sonic-marvell-armhf.bin || make SONIC_CONFIG_BUILD_JOBS=2 INSTALL_DEBUG_TOOLS=y target/sonic-marvell-armhf.bin || make SONIC_CONFIG_BUILD_JOBS=1 INSTALL_DEBUG_TOOLS=y target/sonic-marvell-armhf.bin
 mv target/sonic-marvell-armhf.bin target/sonic-marvell-armhf-dbg.bin
-make SONIC_CONFIG_BUILD_JOBS=1 target/sonic-marvell-armhf.bin
+make SONIC_CONFIG_BUILD_JOBS=4 target/sonic-marvell-armhf.bin || make SONIC_CONFIG_BUILD_JOBS=2 target/sonic-marvell-armhf.bin || make SONIC_CONFIG_BUILD_JOBS=1 target/sonic-marvell-armhf.bin
 sudo docker -H unix:///var/run/march/docker.sock system prune -f
 '''
  }