This covers the issue at installation time, and should be workable to resize even after the installation and the first boot. After the coreos installer runs, the layout of the partitions will include something like:

└ # gdisk -l /dev/sda
GPT fdisk (gdisk) version 1.0.10

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sda: 1000215216 sectors, 476.9 GiB
Model: HUGWORLD SSD 512
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): 9E151132-C6B9-4E44-92FD-60DB86CCB8BE
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 1000215182
Partitions will be aligned on 16-sector boundaries
Total free space is 2669 sectors (1.3 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048            4095   1024.0 KiB  EF02  BIOS-BOOT
   2            4096          264191   127.0 MiB   EF00  EFI-SYSTEM
   3          264192          786432   384.4 MiB   8300  boot
   4          786833         9717247   2.4 GiB     8300  root

The root partition will be resized to its maximum size, after the reboot. This is the best time to shift it to the right and resize the boot partition.

Let’s assume you want to add 512M to the boot partition. We need to move the left boundary of the root partition to the right by 512M, and then resize the boot partition to the right by 512M.

Let’s calculate the new initial sector for the root partition:

512MB = 512 * 1024 * 1024 [B] / 512 [B/sector] = 1048576 sectors
NewSectorForRootPartition = 786833 + 1048576 = 1835409

At this point we can move the root partition to the right by 512M and resize the boot partition to the right by 512M:

echo "<NewSectorForRootPartition>, 4G" | sfdisk --move-data /dev/sda -N 4 # 4 is the root partition, on the right of the boot one to be resized
echo ", +" | sfdisk /dev/sda -N 3 # 3 is the boot partition. We will resize it to the right, until possible (512M)

e2fsck -f /dev/sda3
resize2fs /dev/sda3 # Resize the file system