openjdk: Create a common inc file between openjdk 6 and 7

we hit below errors

ERROR: The function get_jdk_arch defined in openjdk-7-common.inc was
already declared in openjdk-6-common.inc.
BitBake has a global python function namespace so shared functions
should be declared in a common include file
rather than being duplicated, or if the functions are different, please
use different function names.
ERROR: The function get_llvm_configure_arch defined in
openjdk-7-common.inc was already declared in openjdk-6-c
ommon.inc. BitBake has a global python function namespace so shared
functions should be declared in a common in
clude file rather than being duplicated, or if the functions are
different, please use different function names

This patch fixes the above

Signed-off-by: Khem Raj <raj.khem@gmail.com>