[build] add an environment variable to run make reset unattended (#12207)

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
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

@@ -56,6 +56,10 @@
 #  * GZ_COMPRESS_PROGRAM: Select pigz to reduce build time
 #  *                 Default: gzip
 #  *                 Values: pigz,gzip
+#  * UNATTENDED: Don't wait for interactive input from terminal, setting this
+#  *             value to anything will enable it
+#  *             Default: unset
+#  *             Value: y
 #
 ###############################################################################
 
@@ -611,8 +615,13 @@ 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*;