scripts/config: properly report and set string options

Currently, scripts/config removes the leading double-quote from
string options, but leaves the trailing double-quote.

Also, double-quotes in a string are escaped, but scripts/config
does not unescape those when printing

Finally, scripts/config does not escape double-quotes when setting
string options.

Eg. the current behavior:
    $ grep -E '^CONFIG_FOO=' .config
    CONFIG_FOO="Bar \"Buz\" Meh"
    $ ./scripts/config -s FOO
    Bar \"Buz\" Meh"
    $ ./scripts/config --set-str FOO 'Alpha "Bravo" Charlie'
    $ grep -E '^CONFIG_FOO=' .config
    CONFIG_FOO="Alpha "Bravo" Charlie"

Fix those three, giving this new behavior:
    $ grep -E '^CONFIG_FOO=' .config
    CONFIG_FOO="Bar \"Buz\" Meh"
    $ ./scripts/config -s FOO
    Bar "Buz" Meh
    $ ./scripts/config --set-str FOO 'Alpha "Bravo" Charlie'
    $ grep -E '^CONFIG_FOO=' .config
    CONFIG_FOO="Alpha \"Bravo\" Charlie"
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/config

@@ -107,7 +107,8 @@ while [ "$1" != "" ] ; do
 		;;
 
 	--set-str)
-		set_var "CONFIG_$ARG" "CONFIG_$ARG=\"$1\""
+		# sed swallows one level of escaping, so we need double-escaping
+		set_var "CONFIG_$ARG" "CONFIG_$ARG=\"${1//\"/\\\\\"}\""
 		shift
 		;;
 
@@ -124,9 +125,11 @@ while [ "$1" != "" ] ; do
 			if [ $? != 0 ] ; then
 				echo undef
 			else
-				V="${V/CONFIG_$ARG=/}"
-				V="${V/\"/}"
-				echo "$V"
+				V="${V/#CONFIG_$ARG=/}"
+				V="${V/#\"/}"
+				V="${V/%\"/}"
+				V="${V/\\\"/\"}"
+				echo "${V}"
 			fi
 		fi
 		;;