revise build.sh with updated help message

This patch revises build.sh using only one target argument.
It also synchronizes help message with code.

build.sh

@@ -132,7 +132,8 @@ function cleanup_build() {
 }
 
 function usage() {
-  echo "Usage: $0 build/clean"
+  echo "Usage: $0 <target>\n"
+  echo "Here <target> can be: linux, opensbi, uboot, rootfs, img, linux_opensbi_uboot, all or clean."
 }
 
 function fault() {
@@ -141,34 +142,31 @@ function fault() {
 }
 
 function main() {
+  echo $#
   if [[ $# < 1 ]]; then
     fault
   fi
 
-  if [ "$1" = "build" ]; then
-    if [ "$2" = "linux" ]; then
-      build_linux
-    elif [ "$2" = "opensbi" ]; then
-      build_opensbi
-    elif [ "$2" = "uboot" ]; then
-      build_uboot
-    elif [ "$2" = "rootfs" ]; then
-      build_rootfs
-    elif [ "$2" = "img" ]; then
-      build_img
-    elif [ "$2" = "linux_opensbi_uboot" ]; then
-      build_linux
-      build_opensbi
-      build_uboot
-    elif [ "$2" = "all" ]; then
-      build_linux
-      build_opensbi
-      build_uboot
-      build_rootfs
-      build_img
-    else
-      fault
-    fi
+  if [ "$1" = "linux" ]; then
+    build_linux
+  elif [ "$1" = "opensbi" ]; then
+    build_opensbi
+  elif [ "$1" = "uboot" ]; then
+    build_uboot
+  elif [ "$1" = "rootfs" ]; then
+    build_rootfs
+  elif [ "$1" = "img" ]; then
+    build_img
+  elif [ "$1" = "linux_opensbi_uboot" ]; then
+    build_linux
+    build_opensbi
+    build_uboot
+  elif [ "$1" = "all" ]; then
+    build_linux
+    build_opensbi
+    build_uboot
+    build_rootfs
+    build_img
   elif [ "$1" = "clean" ]; then
     cleanup_build
   else