refactor(stm32mp1): split compilations per boot devices

To reduce BL2 size and leave bigger room for BL32, BL2 is compiled
for a given boot device. The default config keeps SD-card and eMMC,
as they use the same MMC framework.
Add the required files to tf-l1-build-plat.
Remove bash line from group/tf-l1-build-plat/stm32mp1-default:nil.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I0f5fddabcfde34b156915c2d0d375bd1b7d2a889
diff --git a/tf_config/stm32mp1-spi-nand b/tf_config/stm32mp1-spi-nand
new file mode 100644
index 0000000..4941e3a
--- /dev/null
+++ b/tf_config/stm32mp1-spi-nand
@@ -0,0 +1,7 @@
+AARCH32_SP=sp_min
+ARCH=aarch32
+ARM_ARCH_MAJOR=7
+CROSS_COMPILE=arm-none-eabi-
+PLAT=stm32mp1
+STM32MP_SPI_NAND=1
+DTB_FILE_NAME=stm32mp157c-ev1.dtb