trunks: Remove default value for BASE_VER

BASE_VER must match the recent version of libchrome being used.
Having it set to a dummy default version that doesn't match the revision
number of libchrome doesn't help anyone much.

Removed the default value and added an error message instructing
the user to set BASE_VER to correct revision number if not already
set (which is the case in normal builds, where BASE_VER comes from
libchrome.eclass).

BUG=None
TEST=emerge-link trunks

Change-Id: I937d9fbb4b0a468d601ce65ffe3c6b6ccde47756
Reviewed-on: https://chromium-review.googlesource.com/264119
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/tss/Makefile b/tss/Makefile
index c771cde..ceab1ef 100644
--- a/tss/Makefile
+++ b/tss/Makefile
@@ -19,7 +19,10 @@
 INSTALL_DATA = $(INSTALL) -m 0644
 LIB_DIR ?= /usr/lib
 
-BASE_VER ?= 307740
+ifeq ($(strip $(BASE_VER)),)
+$(error BASE_VER is not set. Make sure you set it to the correct revision number of libchrome)
+endif
+
 PC_DEPS = dbus-c++-1 libchrome-$(BASE_VER) libchromeos-$(BASE_VER)
 PC_CFLAGS := $(shell $(PKG_CONFIG) --cflags $(PC_DEPS))
 PC_LIBS := $(shell $(PKG_CONFIG) --libs $(PC_DEPS))