vendor: soong: Accept back a value for TARGET_RECOVERY_OVERSCAN_PERCENT

* If the cflags doesn't have value, it may lead to an incomplete display.

Test:
    Before fixed: I:Scaling theme width 0.981481x and height 1.169271x, offsets x: 0 y: 107 w: 0 h: -107
    After fixed:  I:Scaling theme width 1.000000x and height 1.194271x, offsets x: 0 y: 107 w: 0 h: -107
Change-Id: I267290748f2896662696c781a83e141b771e3e2e
Signed-off-by: Col_or <color597@outlook.com>

build/soong/Android.bp

@@ -270,7 +270,7 @@ twrp {
             cflags: ["-DTWRP_EVENT_LOGGING"]
         },
         target_recovery_overscan_percent: {
-            cflags: ["-DOVERSCAN_PERCENT"]
+            cflags: ["-DOVERSCAN_PERCENT=%s"]
         },
         tw_screen_blank_on_boot: {
             cflags: ["-DTW_SCREEN_BLANK_ON_BOOT"]