sdk.py: fix write target sdk manifest failed based on ipk

sdk.py: fix write target sdk manifest failed based on ipk

bitbake meta-toolchain
ls tmp/deploy/sdk/poky-glibc-x86_64-meta-toolchain-i586-toolchain-1.7.manifest -al
...
|-rw-rw-r-- 1 root root 0 Oct 10 15:05 tmp/deploy/sdk/poky-glibc-x86_64-
meta-toolchain-i586-toolchain-1.7.manifest
...

The manifest is empty, the reason is target's ipk config path is
d.getVar('IPKGCONF_TARGET') rather than d.getVar('IPKGCONF_Target')

(From OE-Core rev: 81b3cc448f040dcb4c2f2b05983231ac53270663)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>