classes/populate_sdk_ext: exclude image tasks from locked signatures

classes/populate_sdk_ext: exclude image tasks from locked signatures

Tasks for image recipes cannot be locked and should be excluded from eSDK
generated locked-sigs.inc. get_sdk_install_targets() was not returning right
image targets to be excluded incase of 'minimal' sdk. This change fixes the issue.

(From OE-Core rev: 46401034f017d234833997d2fb3122190f9029bf)

Signed-off-by: Amarnath Valluri <amarnath.valluri@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>