gptboot.efi: align secbuf to 4K

The u-boot EFI implementation of the ReadBlocks and WriteBlocks methods
requires that the provided buffer meet the IO alignment requirements of
the underlying disk. Unlike loader.efi, gptboot.efi doesn't check this
requirement, and therefore fails to perform a successful read. Adjust
secbuf's alignment to 4K in hopes that we will always meet this
requirement.

Reviewed by: imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25111