Write the missing system build props to recovery build prop file.

This was missed by [1], which changed the rules for writing recovery
build prop file.

[1] commit 139c7270

Bug: 110380063
Bug: 112247573
Test: `m bootimage` with aosp_marlin-userdebug. Check the generated
      recovery build prop file against the one before [1].
Change-Id: Ib5829b01a63eee8e8a09486dd68063ba297b7443