RPi3: add README

Building, using, etc.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>

build49

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+#
+# Adjust to taste.
+#
+# This assumes the following directory structure:
+#
+# \
+# \edk2
+# \RaspberryPiPkg
+# \build49
+#
+
+export WORKSPACE=$PWD
+export PACKAGES_PATH=$WORKSPACE/edk2:$WORKSPACE/RaspberryPiPkg
+export EDK_TOOLS_PATH=$WORKSPACE/edk2/BaseTools
+export GCC49_AARCH64_PREFIX=aarch64-linux-
+export CROSS_COMPILE=aarch64-linux-
+
+rm -rf Conf
+mkdir Conf
+make -C $EDK_TOOLS_PATH
+echo $EDK_TOOLS_PATH
+. $WORKSPACE/edk2/edksetup.sh BaseTools
+
+pushd $WORKSPACE/RaspberryPiPkg
+BUILD_COMMIT=`git rev-parse --short HEAD`
+popd
+BUILD_DATE=`date +%m/%d/%Y`
+
+build -b DEBUG -a AARCH64 -t GCC49 -p RaspberryPiPkg/RaspberryPiPkg.dsc -DBUILD_DATE=$BUILD_DATE -DBUILD_COMMIT=$BUILD_COMMIT
+build -b RELEASE -a AARCH64 -t GCC49 -p RaspberryPiPkg/RaspberryPiPkg.dsc -DBUILD_DATE=$BUILD_DATE -DBUILD_COMMIT=$BUILD_COMMIT

build5

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+#
+# Adjust to taste.
+#
+# This assumes the following directory structure:
+#
+# \
+# \edk2
+# \RaspberryPiPkg
+# \gcc-linaro-5.5.0-2017.10-i686_aarch64-linux-gnu
+# \build5
+#
+
+export WORKSPACE=$PWD
+export PACKAGES_PATH=$WORKSPACE/edk2:$WORKSPACE/RaspberryPiPkg
+export EDK_TOOLS_PATH=$WORKSPACE/edk2/BaseTools
+export PATH=$PATH:$PWD/gcc-linaro-5.5.0-2017.10-i686_aarch64-linux-gnu/bin
+export GCC5_AARCH64_PREFIX=aarch64-linux-gnu-
+
+rm -rf Conf
+mkdir Conf
+make -C $EDK_TOOLS_PATH
+echo $EDK_TOOLS_PATH
+. $WORKSPACE/edk2/edksetup.sh BaseTools
+
+pushd $WORKSPACE/RaspberryPiPkg
+BUILD_COMMIT=`git rev-parse --short HEAD`
+popd
+BUILD_DATE=`date +%m/%d/%Y`
+
+build -b DEBUG -a AARCH64 -t GCC5 -p RaspberryPiPkg/RaspberryPiPkg.dsc -DBUILD_DATE=$BUILD_DATE -DBUILD_COMMIT=$BUILD_COMMIT
+build -b RELEASE -a AARCH64 -t GCC5 -p RaspberryPiPkg/RaspberryPiPkg.dsc -DBUILD_DATE=$BUILD_DATE -DBUILD_COMMIT=$BUILD_COMMIT