isoboot, gptboot: Fix WITHOUT_LOADER_GELI (gptboot) and isoboot in general

gptboot was broken when r316078 added the LOADER_GELI_SUPPORT #ifdef to
not pass geliargs via exec. KARGS_FLAGS_EXTARG must not be used if we're
not going to pass an additional argument to exec.

PR: 228151
Submitted by: guyyur@gmail.com
MFC after: 1 week