Add IMEI column to subscriber table

Extend the database scheme, add imei to the hlr_subscriber struct and
create db_subscr_update_imei_by_imsi() and db_subscr_get_by_imei(). The
new functions are used in db_test, and in follow-up commits [1], [2].

Upgrade DB schema to version 2. SQLite can only insert new columns at
the end of the table, so this happens when upgrading the database. In
new databases, the column is placed after the IMEISV column (where it
makes more sense in my opinion). This should not have any effect, as
we never rely on the order of the columns in the tables.

Follow-up commit [1] will make use of this column to save the IMEI as
received from the MSC/VLR with the Check-IMEI Procedure. It was
decided to use Check-IMEI instead of the recent Automatic Device
Detection Procedure (which would send the IMEISV) in OS#3733, because
with Check-IMEI we don't need to rely on very recent releases of the
specification.

[1] change-id I09274ecbed64224f7ae305e09ede773931da2a57
    "Optionally store IMEI in subscriber table"
[2] change-id I1af7b573ca2a1cb22497052665012d9c1acf3b30
    "VTY: integrate IMEI"

Depends: Id2d2a3a93b033bafc74c62e15297034bf4aafe61 (libosmocore)
Related: OS#2541
Change-Id: If232c80bea35d5c6864b889ae92d477eeaa3f45d

sql/hlr.sql

@@ -5,8 +5,10 @@ CREATE TABLE subscriber (
 	imsi		VARCHAR(15) UNIQUE NOT NULL,
 	-- Chapter 2.1.2
 	msisdn		VARCHAR(15) UNIQUE,
-	-- Chapter 2.2.3: Most recent / current IMEI
+	-- Chapter 2.2.3: Most recent / current IMEISV
 	imeisv		VARCHAR,
+	-- Chapter 2.1.9: Most recent / current IMEI
+	imei		VARCHAR(14),
 	-- Chapter 2.4.5
 	vlr_number	VARCHAR(15),
 	-- Chapter 2.4.6
@@ -74,4 +76,4 @@ CREATE UNIQUE INDEX idx_subscr_imsi ON subscriber (imsi);
 
 -- Set HLR database schema version number
 -- Note: This constant is currently duplicated in src/db.c and must be kept in sync!
-PRAGMA user_version = 1;
+PRAGMA user_version = 2;