Increase EFI boot file size frok 128k to 384k

generate_fat.sh does the following:

• create an 800kb zero-filled file
• create an md device backed by this file
• format the device fat12
• mount the filesystem
• create the EFI ESP directory structure
• create the EFI boot file (BOOTx64 for amd64, BOOTaa64 for aarch64, etc)
• Adds a marker to the beginning of the file, and pad it to 384kb
• 384kb was chosen as it is less than half of 800kb, thus allowing users to keep a backup of their older boot file in the small partition
• Unmount the filesystem
• Scan the image and find the offset where the marker was inserted
• The process requires root, to make image generation easier, images for each architecture are pregenerated, compressed with xz, and checked into svn.

The Makefile that generates boot1.efifat does the following:

• Ensure the compiled boot1.efi file is no larger than the generated image
• Decompress the template created by generate-fat.sh
• dd the contents of boot1.efi into boot1.efifat starting at the offset where the marker is found. This allows any file less than the maximum size to be written into the fat filesystem without having to mount it, so no root privileges are required.

Later work by imp and myself makes bsdinstall create a 200mb fat16 instead
of using this process, but it is retained to make image generation easier.

Submitted by: Eric McCorkle (original version)
Reviewed by: emaste, tsoome, Eric McCorkle
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D9680