Before you start:
Instructions assume you are using Linux.
You'll need a DHCP server and a TFTP server. 
These instructions need ethernet so will
only work with the full version of the Peach.

-- Flashing U-boot --

Connect the Peach via usb. Run dmesg and check
that you see the mbed debugger and the serial
port. Make a note of node the serial port was
assigned. It should be ttyACM0 in most cases.

In one shell run:
minicom -D /dev/ttyACM0 <-- Replace if different

In another shell run:
openocd -f renesas_rza1h_swd.cfg

You should see openocd detect the RZ/A1H..

Then in yet another run:
arm-none-eabi-gdb

Once gdb is running enter the following commands:

target remote localhost:3333
monitor target reset halt
load /path/to/u-boot-ram
c

You should now see u-boot start up on the serial
console.

Copy u-boot-spiflash to u-boot at the root of your
tftp server. If your tftp server address is not
192.168.2.1 you need to the following on the u-boot console:
setenv serverip <your tftp ip>

Finally download u-boot from the server and write it to
the spi flash like so:
run writeuboot

If everything worked you should be able to press the reset
button on the Peach and see the u-boot console appear.
If not reload the ram version of u-boot with GDB and try
again.

-- Building the kernel and device and writing them -- 

make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- xipImage
make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- dtbs

copy arch/arm/boot/dts/r7s72100-grpeach.dtb and arch/arm/boot/xipImage

-- Building a rootfs and writing it --

Download buildroot from https://buildroot.org/download.html
Extract buildroot.
Copy buildroot.config to .config in the buildroot dir
Copy busybox.config to busybox.config in the buildroot dir
Run make
Copy output/images/rootfs.squashfs to the root of your tftp server as root.romfs
In u-boot run "run writerootfs"