ro.zygote in base_system.mk is optional

The setting of ro.zygote in base_system.mk is optional, which means the
value can be overriden by other (probably more specific) mk files.

Bug: 117892318
Bug: 158735147
Test: atest --host post_process_prop_unittest

Exempt-From-Owner-Approval: cherry-pick from master

Merged-In: Ia7a67c0a04fad343d6591417f40dd4b9ddadc5e4
(cherry picked from commit b1261aac333e9980842af1120a4af747130e9f03)
Change-Id: Ia7a67c0a04fad343d6591417f40dd4b9ddadc5e4
diff --git a/target/product/base_system.mk b/target/product/base_system.mk
index bec550b..f6770fb 100644
--- a/target/product/base_system.mk
+++ b/target/product/base_system.mk
@@ -348,7 +348,7 @@
 endif
 
 PRODUCT_COPY_FILES += system/core/rootdir/init.zygote32.rc:system/etc/init/hw/init.zygote32.rc
-PRODUCT_SYSTEM_PROPERTIES += ro.zygote=zygote32
+PRODUCT_SYSTEM_PROPERTIES += ro.zygote?=zygote32
 
 PRODUCT_SYSTEM_PROPERTIES += debug.atrace.tags.enableflags=0