tune-*: define more generic DEFAULTTUNE to share feed between machines

tune-*: define more generic DEFAULTTUNE to share feed between machines

* this is mostly for backwards compatibility and to share binary feed
  like it was before, but now without missing different -mtune in it
* if you want to build some package with -mtune add something like this
  to your distro config
  DEFAULTTUNE_qemuarm_pn-openssl = "arm926ejs"
  DEFAULTTUNE_qemuarmx_pn-openssl = "xscale"
  be aware that if you do this you should do it also for all packages
  which depends on openssl because if you dont and you build e.g. dhcp,
  then dhcp build for arm926ejs (even with DEFAULTTUNE armv5te) will
  depend on openssl with arm926ejs, so dhcp in armv5te feed will be
  rebuild after each MACHINE switch.
* cortexm3, cortexr4, iwmmx and ep9312 are using own DEFAULTTUNE because
  they define also different -march
* shared feeds are
  armv4t: arm920t, arm9tdmi
  armv5te: arm926ejs, xscale
  armv7a-neon: cortexa8, cortexa9

(From OE-Core rev: a11bdc36a1be18cc5aa14682b2a2c9ee83141f51)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>