Hide fake storage behind env var

Makefile

@@ -27,6 +27,9 @@ SHELL = /bin/bash
 ifeq ("${DEBUG}","1")
 	BUILD_TAGS := ${BUILD_TAGS},debug
 endif
+ifeq ("${FAKE_STORAGE}", "1")
+	BUILD_TAGS := ${BUILD_TAGS},fake_storage
+endif
 
 APP := ""
 TEXT_START = 0x80010000 # ramStart (defined in mem.go under relevant tamago/soc package) + 0x10000

README.md

@@ -142,6 +142,16 @@ the `imx_boot` tool, or similar.
 Note that since this OS image is not being loaded via the bootloader, it does not need
 to be added to the FT log.
 
+#### Optional development environment variables
+
+The following environment variables may be set when compiling the Trusted OS image in order
+to add extra debugging support to the resulting image:
+
+| Variable         | Description
+|------------------|------------
+| `DEBUG`          | When set to `1`, enables output of logging, stdout, etc. See #debugging section below for more details
+| `FAKE_STORAGE`   | When set to `1`, provides a storage card implementation which passes-through to SDCard on native hardware, but uses RAM to emulate an SDCard otherwise.
+
 ### Encrypted RAM support
 
 Only on i.MX6UL P/Ns, the `BEE` environment variable must be set to match
@@ -182,6 +192,13 @@ The Trusted OS image can be executed under emulation as follows:
 make qemu
 ```
 
+Note that emulated MMC storage is not currently supported, so the Trusted OS should
+be built with the `FAKE_STORAGE` environment variable set to `1`:
+
+```bash
+make DEBUG=1 FAKE_STORAGE=1 trusted_os
+```
+
 The emulation run network connectivity should be configured as follows (Linux
 example with tap0):
 

trusted_os/storage.go

@@ -12,8 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build !debug
-// +build !debug
+//go:build !fake_storage
+// +build !fake_storage
 
 package main
 

trusted_os/storage_debug.go

@@ -12,8 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-//go:build debug
-// +build debug
+//go:build fake_storage
+// +build fake_storage
 
 package main