wic/efi-bootdisk.wks: Fix for 6.1 kernel versions

oe-selftest efibootpartition.GenericEFITest.test_boot_efi was failing for 6.1 kernels with:

| ERROR: _exec_cmd: export PATH=[...] mcopy -i [...]/rootfs_boot.1.vfat -s [...]/rootfs1/* ::/ returned '1' instead of 0
| output: Disk full

I believe we hit a file boundary size and having "0" overhead in the image meant
the files couldn't be installed. Allow a small amount of overhead to avoid
the error.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>