You shouldn't use the platformsdk option when building the ANE but when building your application using an ANE. The option is invalid for a native extension build.
When you are building your iOS application you should specify it as below:
adt -package \
-target ipa-test \
-provisioning-profile /path_to_provisioning_profile/Provisioning_Profile.mobileprovision \
-storetype pkcs12 \
-keystore /path_to_certificate/certificate.p12 \
-storepass xxxxx \
myApp.ipa \
application_xml_file.xml \
application_swf.swf \
-platformsdk PATH_TO_SDK
When you are building your ANE you should make sure you have shared libraries enabled and specify the frameworks your extension is using in the iOS platform options xml file.
adt -package \
-storetype pkcs12 -keystore /path_to_certificate/certificate.p12 -storepass xxxxx \
-target ane extension.ane extension.xml \
-swc extenion_as_library.swc \
-platform iPhone-ARM -platformoptions platform.xml -C PATH_TO_IOS_LIB . \
-platform default -C PATH_TO_DEFAULT_LIB .