Fix config checks for Image Builder and SDK - only build them when requested.

Fix config checks for Image Builder and SDK - only build them when requested.
Clear all packages from bin/packages at the beginning of package/compile when the Image Builder is selected, because the Image Builder can't handle multiple versions of the same package.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@6102 3c298f89-4303-0410-b956-a3cf2f4a3e73