sysmocom-idu: Fix the core2 vs. core2-32 tune selection

The distro is included after the machine so we can not rely
on the DISTRO_VERSION here. Use the AVAILTUNES and see if the
core2-32 is listed and then pick it.

conf/machine/sysmocom-idu.conf

@@ -19,7 +19,8 @@ KERNEL_IMAGETYPE = "bzImage"
 IMAGE_FSTYPES = "ext4"
 
 # After dora core2 got renamed to core2-32
-DEFAULTTUNE="${@['core2-32','core2'][d.getVar('DISTRO_VERSION', True)[0:3] == '1.5']}"
+# After dora core2 got renamed to core2-32
+DEFAULTTUNE := "${@['core2', 'core2-32']['core2-32' in d.getVar('AVAILTUNES', True)]}"
 
 SERIAL_CONSOLE = "115200 ttyS0"
 MACHINE_CONSOLE = "console=ttyS0,115200n8"