bitbake.conf: Simplify TARGET_SYS/SDK_SYS

As pointed out by Masahiro Yamada <masahiroy@kernel.org>, ('' or 'custom')
equates to "custom" and this code seems unecesaarily complicated and unused.

It was likely intended to supress the value if XXX_OS was set to '' but the
code doesn't actually do that. Simplify it as we shouldn't have this indirection
if we don't need it, its horrible.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>