dfu: add 'SCRIPT' entity

Define a new 'SCRIPT' type for DFU entities. The downloaded data are
treated as simple u-boot's scripts and executed with run_command_list()
function.

Flashing the 'SCRIPT' entity might result in changing the 'dfu_alt_info'
environment variable from the flashed script, so add a global variable
for tracking the potential need to reinitialize the dfu_alt_info related
structures.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

doc/README.dfu

@@ -17,7 +17,7 @@ Overview:
   - The access to mediums is done in DFU backends (driver/dfu)
 
   Today the supported DFU backends are:
-  - MMC (RAW or FAT / EXT2 / EXT3 / EXT4 file system / SKIP)
+  - MMC (RAW or FAT / EXT2 / EXT3 / EXT4 file system / SKIP / SCRIPT)
   - NAND
   - RAM
   - SF (serial flash)
@@ -92,6 +92,7 @@ Commands:
       <name> fat <dev> <part_id> [mmcpart <num>]   file in FAT partition
       <name> ext4 <dev> <part_id> [mmcpart <num>]  file in EXT4 partition
       <name> skip 0 0                              ignore flashed data
+      <name> script 0 0                            execute commands in shell
 
       with <partid> being the GPT or DOS partition index,
       with <num> being the eMMC hardware partition number.
@@ -116,6 +117,20 @@ Commands:
 
       "u-boot-<board1>.bin raw 0x80 0x800; u-boot-<board2>.bin skip 0 0"
 
+    When flashing new system image requires do some more complex things
+    than just writing data to the storage medium, one can use 'script'
+    type. Data written to such entity will be executed as a command list
+    in the u-boot's shell. This for example allows to re-create partition
+    layout and even set new dfu_alt_info for the newly created paritions.
+    Such script would look like:
+	--->8---
+	setenv dfu_alt_info ...
+	setenv mbr_parts ...
+	mbr write ...
+	--->8---
+    Please note that this means that user will be able to execute any
+    arbitrary commands just like in the u-boot's shell.
+
   "nand" (raw slc nand device)
     cmd: dfu 0 nand <dev>
     each element in "dfu_alt_info" =