Introduce build script for arm-none-eabi cross-compilations

This is a necessary step to use libosmocore-arm dep in OsmocomBB.

After this patch is submitted the "./contrib/jenkins.sh" line in
libosmocore jenkins jobs [1][2] needs to be replaced with:

  if [[ "$JOB_NAME" == *"arch=arm-none-eabi,label=linux_amd64_debian8"* ]]
  then
    ./contrib/jenkins-arm.sh
  else
    # (FreeBSD && debian8)_amd64 builds
    ./contrib/jenkins.sh
  fi

Furthermore, the "arch" matrix-build axis has to be created,
holding "arm-none-eabi" and "amd64". Following combination filter 
is necessary to skip cross-compilations on FreeBSD buildslave:

  !(arch=="arm-none-eabi" && label=="FreeBSD_amd64")

[1] https://jenkins.osmocom.org/jenkins/job/libosmocore/
[2] https://jenkins.osmocom.org/jenkins/job/libosmocore-gerrit/

Change-Id: I0db83ad2c94ada65c00fd1bcf867830dec80085d

contrib/jenkins-arm.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+
+set -ex
+
+./contrib/verify_value_string_arrays_are_terminated.py $(find . -name "*.[hc]")
+
+autoreconf --install --force
+./configure --enable-static \
+	--prefix=/usr/local/arm-none-eabi \
+	--host=arm-none-eabi \
+	--enable-embedded \
+	--disable-shared \
+	CFLAGS="-Os -ffunction-sections -fdata-sections -nostartfiles -nodefaultlibs"
+
+$MAKE $PARALLEL_MAKE \
+	|| cat-testlogs.sh
+
+# verify build in dir other than source tree
+rm -rf *
+git checkout .
+autoreconf --install --force
+mkdir builddir
+cd builddir
+
+../configure --enable-static \
+	--prefix=/usr/local/arm-none-eabi \
+	--host=arm-none-eabi \
+	--enable-embedded \
+	--disable-shared \
+	CFLAGS="-Os -ffunction-sections -fdata-sections -nostartfiles -nodefaultlibs"
+
+$MAKE $PARALLEL_MAKE \
+	|| cat-testlogs.sh