mimo-updater: use ${SYSROOT}/usr/share/libchrome/BASE_VER

BASE_VER will be removed from environment variable.

$(SRC)/../common-mk/BASE_VER is also a bad path, even common-mk/BASE_VER
is not removed yet. Since this software doesn't include common-mk at
all. USE $(SYSROOT)/usr/share/libchrome/BASE_VER instead.

BUG=chromium:1062979
TEST=emerge

Change-Id: I633ff1fbbd4384b4b823a18b7ee89a31ca2c579f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/mimo-updater/+/2331971
Reviewed-by: Kyle Williams <kdgwill@chromium.org>
Tested-by: Kyle Williams <kdgwill@chromium.org>
Commit-Queue: Qijiang Fan <fqj@google.com>
diff --git a/common.mk b/common.mk
index 206bfa4..320a132 100644
--- a/common.mk
+++ b/common.mk
@@ -124,10 +124,8 @@
 export SRC ?= $(CURDIR)
 
 # If BASE_VER is not set, read the libchrome revision number from
-# common-mk/BASE_VER file.
-ifeq ($(strip $(BASE_VER)),)
-BASE_VER := $(shell cat $(SRC)/../common-mk/BASE_VER)
-endif
+# /usr/share/libchrome/BASE_VER.
+BASE_VER := $(shell cat $(SYSROOT)/usr/share/libchrome/BASE_VER)
 $(info Using BASE_VER=$(BASE_VER))
 
 # Re-start in the $(OUT) directory if we're not there.