procd: document nand.sh a bit

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

SVN-Revision: 43372

package/system/procd/files/nand.sh

@@ -201,6 +201,7 @@ nand_do_upgrade_success() {
 	reboot -f
 }
 
+# Flash the UBI image to MTD partition
 nand_upgrade_ubinized() {
 	local ubi_file="$1"
 	local mtdnum="$(find_mtd_index "$CI_UBIPART")"
@@ -223,6 +224,7 @@ nand_upgrade_ubinized() {
 	nand_do_upgrade_success
 }
 
+# Write the UBIFS image to UBI volume
 nand_upgrade_ubifs() {
 	local rootfs_length=`(cat $1 | wc -c) 2> /dev/null`
 
@@ -269,6 +271,7 @@ nand_upgrade_tar() {
 	nand_do_upgrade_success
 }
 
+# Recognize type of passed file and start the upgrade process
 nand_do_upgrade_stage2() {
 	local file_type=$(identify $1)
 
@@ -319,6 +322,19 @@ nand_upgrade_stage1() {
 }
 append sysupgrade_pre_upgrade nand_upgrade_stage1
 
+# Check if passed file is a valid one for NAND sysupgrade. Currently it accepts
+# 3 types of files:
+# 1) UBI - should contain an ubinized image, header is checked for the proper
+#    MAGIC
+# 2) UBIFS - should contain UBIFS partition that will replace "rootfs" volume,
+#    header is checked for the proper MAGIC
+# 3) TRX - archive has to include "sysupgrade-BOARD" directory with a non-empty
+#    "CONTROL" file (at this point its content isn't verified)
+#
+# You usually want to call this function in platform_check_image.
+#
+# $(1): board name, used in case of passing TRX file
+# $(2): file to be checked
 nand_do_platform_check() {
 	local board_name="$1"
 	local tar_file="$2"