Update README.md

To reflect recent change to block device
firmware storage

Author: LiyouZhou

README.md

@@ -23,6 +23,10 @@ User **must** set in `mbed_app.json`:
 1. `firmware_metadata_header_address`, Offset at which the metadata header of the active firmware is written. **Must align to flash erase boundary**
 1. `firmware_metadata_header_size`, Size of the active metadata header region. The application starts at `firmware_metadata_header_address + firmware_metadata_header_size`. **Must align to vector table size boundary and flash erase boundary**
 1. `update-client.application-details`, This is the same as `firmware_metadata_header_address` but needs to be configured to allow the dependency module of bootloader to pick up this configuration.
+1. `update-client.storage-address`, The offset from the begining of sd block device where the firmware candidates are stored.
+1. `update-client.storage-size`, total size on the block device reserved for firmware storage
+
+All these configurations must be set the same in the mbed cloud client when compiling the corresponding application for successful update operation.
 
 User **may** set in `mbed_app.json`:
 1. `MAX_COPY_RETRIES`, The number of retries after a failed copy attempt.
@@ -67,13 +71,29 @@ User **may** set in `mbed_app.json`:
 
 ## External Storage
 
-The firmware update candidates are currently stored on an external sd card with a FAT32 file system. The expected folder structure is as follows:
+The firmware update candidates are currently stored on an external sd card. The firmware is stored sequencially on the block device. The expected layout is as follows:
 ```
-SDRoot
-  +-- firmware
-        +-- header_0.bin
-        +-- firmware_0.bin
-        +-- header_1.bin
-        +-- firmware_1.bin
-        +-- ...
+    +--------------------------+<-+ End of SD card block device
+    |                          |
+    +--------------------------+<-+ update-client.storage-size + update-client.storage-address
+    |                          |
+    +--------------------------+ 
+    |                          |
+    |   Firmware Candidate 1   |
+    |                          |
+    +--------------------------+
+    |   Firmware Candidate 1   |
+    |     Metadata Header      |
+    +--------------------------+
+    |                          |
+    +--------------------------+
+    |                          |
+    |   Firmware Candidate 0   |
+    |                          |
+    +--------------------------+
+    |   Firmware Candidate 0   |
+    |     Metadata Header      |
+    +--------------------------+ <-+ update-client.storage-address
+    |                          |
+    +--------------------------+ <-+ Start of SD card block device (ie 0x0)
 ```