build: add an env var to run make reset unattended (#13821)

previously "make reset" was expecting user input from the terminal to do its job
setting UNATTENDED to any non-zero string will allow "make reset" to run without interactive confirmation

- Why I did it
This is the backport to 202211 from master (see PR #12207)
When doing automated builds of SONiC images, we need to reset the working repositories between each build.

- How I did it
Adding an environment variable that is read by Makefile.work

- How to verify it
running
UNATTENDED=1 make reset 
should make an automatic reset of all working directories

Makefile.work

@@ -53,6 +53,10 @@
 #  * ENABLE_BOOTCHART: Enable SONiC bootchart
 #  *                 Default: n
 #  *                 Values: y,n
+#  * UNATTENDED: Don't wait for interactive input from terminal, setting this
+#  *             value to anything will enable it
+#  *             Default: unset
+#  *             Value: y
 #
 ###############################################################################
 
@@ -585,8 +589,13 @@ init :
 
 .ONESHELL : reset
 reset :
-	$(Q)echo && echo -n "Warning! All local changes will be lost. Proceed? [y/N]: "
-	$(Q)read ans && (
+	$(Q)echo && (
+	if [ -z "$(UNATTENDED)" ]; then
+	   echo -n "Warning! All local changes will be lost. Proceed? [y/N]: "
+	   @read ans
+	else
+	   ans=y
+	fi
 	if [ $$ans == y ]; then
 	    echo "Resetting local repository. Please wait...";
 	    sudo rm -rf fsroot*;