STM32MP15C-ED1 is the child board plugged on EV1 mother board. ED1 can
be used without its mother board with some interface restrictions
(I.e. no USB-C). This change defines the platform flavor. The several
components (optee_os, tf-a, u-boot, linux) already support ED1 DTS file
identifier.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Acked-by: Jerome Forissier <jerome@forissier.org>

Build OP-TEE for STM32MP1 based platforms. Currently supports platforms
STM32MP157A-DK1, STM32MP157C-DK2 and STM32MP157C-EV1 using a SDcard as
boot media.

Default build targets board STM32MP157C-DK2:
# make

One can specify a target board with PLATFORM configuration switch:
# make PLATFORM=stm32mp1-157A_DK1
# make PLATFORM=stm32mp1-157C_DK2
# make PLATFORM=stm32mp1-157C_EV1

Refer to br-ext/board/stmicroelectronics/stm32mp1-tz/readme.txt for
mode information.

TF-A, OP-TEE OS, U-Boot and Linux kernel are built outside Builrdoot.
All other embedded components are built through Buildroot.
Buildroot generates the final SDcard binary image file to be loaded
raw in the target SD card. The image includes a GPT partition table.
See br-ext/board/stmicroelectronics/stm32mp1-tz/genimage-*.cfg
for more details.

Note on U-Boot defconfig: from v2019.10 up to v2020.07-rc2, one shall
use stm32mp15_optee_defconfig. From v2020-rc3 onward, one shall use
stm32mp15_trusted_defconfig. For flexibility, stm32mp1.mk supports both.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>