bitbake.conf: set READELF for cross compilation

bitbake.conf: set READELF for cross compilation

In the case of using an external toolchain that supports multilib
compilation with a single binary, TARGET_PREFIX is the same for both main
and multilib abis. Without READELF exported, python3 assumes it is
either the readelf for ${BUILD_SYS}-readelf. Exporting cross readelf
fixes the build issue.

checking LDLIBRARY... libpython$(LDVERSION).so
checking for i586-montavistamllib32-linux-ranlib...
x86_64-montavista-linux-ranlib
checking for i586-montavistamllib32-linux-ar...
x86_64-montavista-linux-ar
checking for i586-montavistamllib32-linux-readelf... no
checking for readelf... readelf
configure: WARNING: using cross tools not prefixed with host triplet

(From OE-Core rev: 3442ee423813d547be7899a25ea31efe719e662f)

Signed-off-by: Jeremy Puhlman <jpuhlman@mvista.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>