Support kernels larger than EFI_STAGING_SIZE in loader.efi

With a very large kernel or module the staging area may be too small to
hold it. When this is the case try to allocate more space before failing
in the efi copyin/copyout/readin functions.

Reviewed by: imp, tsoome
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22569