bitbake.conf: fix ineffective include conf/target/${TARGET_SYS}.conf

bitbake.conf: fix ineffective include conf/target/${TARGET_SYS}.conf

TARGET_SYS is defined in terms of TARGET_ARCH, so it's not valid
until after TUNE_ARCH has been set by the machine config. The
original order of includes resulted in an attempt to include
non-existent files such as:

  conf/target/INVALID-oe-linux.conf

(From OE-Core rev: b33e644da0d8b6edb97257b16430b545c289883a)

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>