configure: Allow override of kdf_do variable.

Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

ChangeLog

@@ -1,3 +1,7 @@
+2021-10-11  NIIBE Yutaka  <gniibe@fsij.org>
+
+	* src/configure (kdf_do): It can be overridden, now.
+
 2021-07-01  NIIBE Yutaka  <gniibe@fsij.org>
 
 	* src/configure (CONFIG): Add KDF configuration.

NEWS

@@ -1,6 +1,34 @@
 Gnuk NEWS - User visible changes
 
 
+* Major changes in Gnuk 1.2.19
+
+  Released 2021-10-1x, by NIIBE Yutaka
+
+** KDF Data Object configuration
+KDF Data Object should be highly recommended for all configurations.
+Nevertheless, for backward compatibillity, in Gnuk 1.2, it is optional
+by default; It is up to user to configure KDF Data Object before
+importing private keys.  In this situatoin, it is not good to
+introduce new option like --enable-always-require-kdf-do, because it
+might wrongly encourage use of Gnuk with no KDF Data Object setting,
+by confusion.  If needed, please run configure:
+
+  kdf_do=required ./configure --enable-factory-reset --target...
+
+or
+
+  kdf_do=optional ./configure --enable-factory-reset --target...
+
+Please note that such a use of variable by shell command line is not
+well supported by the configure script (for other variables), but
+override of kdf_do is needed in some situations.
+
+** Upgrade of Chopstx
+We use Chopstx 1.20.  This enables use with PC/SC for GNU/Linux
+emulation.
+
+
 * Major changes in Gnuk 1.2.18
 
   Released 2021-04-02, by NIIBE Yutaka

src/configure

@@ -46,7 +46,7 @@ hid_card_change=no
 factory_reset=no
 ackbtn_support=yes
 flash_override=""
-kdf_do=optional
+kdf_do=${kdf_do:-optional}
 # For emulation
 prefix=/usr/local
 exec_prefix='${prefix}'
@@ -219,7 +219,7 @@ if test "$target" = "GNU_LINUX"; then
   emulation="yes"
   cross=""
   mcu="none"
-  kdf_do=required
+  kdf_do=${kdf_do:-required}
   def_emulation="-DGNU_LINUX_EMULATION"
   def_memory_size="-DMEMORY_SIZE=1024"
   enable_hexoutput=""