[ImageBuilder] UBOOT_SOURCE/SCRIPTS variables must always be set

[Luca Miccio <luca.miccio@xxxxxxx>]

The current README incorrectly omits the need for the UBOOT_SOURCE and
UBOOT_SCRIPT variables for uboot-script-gen to function properly.
Moreover, the script silently fails if these variables are not set.
Return an error if the the UBOOT_SCRIPT variable is not set and
correct the documentation accordingly.

Signed-off-by: Luca Miccio <luca.miccio@xxxxxxx>
---
 README.md                |  3 ++-
 scripts/uboot-script-gen | 12 ++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index 4fcd908..ae2fdfd 100644
--- a/README.md
+++ b/README.md
@@ -272,7 +272,8 @@ Where:
 
 - UBOOT_SOURCE and UBOOT_SCRIPT specify the output. They are optional
   as you can pass -o FILENAME to uboot-script-gen as a command line
-  parameter
+  parameter. It has to be set either in the config file or CLI argument
+  though.
 
 - APPEND_EXTRA_CMDS: is optional and specifies the path to a text file
   containing extra u-boot commands to be added to the boot script before
diff --git a/scripts/uboot-script-gen b/scripts/uboot-script-gen
index dcf5bdb..b07cbf1 100755
--- a/scripts/uboot-script-gen
+++ b/scripts/uboot-script-gen
@@ -1210,6 +1210,18 @@ then
     UBOOT_SCRIPT="$uboot_out_opt".scr
     UBOOT_SOURCE="$uboot_out_opt".source
 fi
+
+if test ! "$UBOOT_SOURCE"
+then
+    echo "UBOOT_SOURCE not set, either specify it in the config or set it with 
the -o option"
+    exit 1
+fi
+
+if test ! "$UBOOT_SCRIPT"
+then
+    UBOOT_SCRIPT="$UBOOT_SCRIPT".scr
+fi
+
 if test "$fit_opt" && ! test "$FIT"
 then
     FIT="${UBOOT_SOURCE%.source}.fit"
-- 
2.34.1