Add iSCSI support back

* EDK2 commit tianocore/edk2@4ecb1ba removed iSCSI support
* Add it back through the corresponding build option, and use this opportunity
  to factorize the build flags and have DEBUG and RELEASE use the same ones.
* Closes #125

build_firmware.sh

@@ -7,6 +7,7 @@ make -C edk2/BaseTools || exit 1
 export GCC5_AARCH64_PREFIX=aarch64-linux-gnu-
 export WORKSPACE=$PWD
 export PACKAGES_PATH=$WORKSPACE/edk2:$WORKSPACE/edk2-platforms:$WORKSPACE/edk2-non-osi
+export BUILD_FLAGS="-D SECURE_BOOT_ENABLE=TRUE -D INCLUDE_TFTP_COMMAND=TRUE -D NETWORK_ISCSI_ENABLE=TRUE"
 source edk2/edksetup.sh || exit 1
-build -a AARCH64 -t GCC5 -p edk2-platforms/Platform/RaspberryPi/RPi4/RPi4.dsc -b DEBUG --pcd gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVendor=L"https://github.com/pftf/RPi4" --pcd gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVersionString=L"UEFI Firmware $APPVEYOR_REPO_TAG_NAME (DEBUG)" || exit 1
-build -a AARCH64 -t GCC5 -p edk2-platforms/Platform/RaspberryPi/RPi4/RPi4.dsc -b RELEASE --pcd gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVendor=L"https://github.com/pftf/RPi4" --pcd gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVersionString=L"UEFI Firmware $APPVEYOR_REPO_TAG_NAME" -D SECURE_BOOT_ENABLE=TRUE -D INCLUDE_TFTP_COMMAND=TRUE || exit 1
+build -a AARCH64 -t GCC5 -p edk2-platforms/Platform/RaspberryPi/RPi4/RPi4.dsc -b DEBUG --pcd gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVendor=L"https://github.com/pftf/RPi4" --pcd gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVersionString=L"UEFI Firmware $APPVEYOR_REPO_TAG_NAME (DEBUG)" $BUILD_FLAGS || exit 1
+build -a AARCH64 -t GCC5 -p edk2-platforms/Platform/RaspberryPi/RPi4/RPi4.dsc -b RELEASE --pcd gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVendor=L"https://github.com/pftf/RPi4" --pcd gEfiMdeModulePkgTokenSpaceGuid.PcdFirmwareVersionString=L"UEFI Firmware $APPVEYOR_REPO_TAG_NAME" $BUILD_FLAGS || exit 1