fill-in build_system section

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/software_reference/boot_process/spl.md

@@ -1,3 +1,23 @@
+The SPL is the Secondary Program Loader (the Primary Program Loader
+being the code in the [Boot ROM][1]), it is the first
+user-customizable piece of code run on the CPU.
+
+The SPL is built as part of the U-Boot bootlader. Actually, it shares
+most of its code with it.
+
+The SPL is loaded by the Boot ROM at address `0x00000000` in SRAM A1
+and C, and has a maximum size of 32KB. It contains a "**BOOT0
+Header**" that has been checked previously by the Boot ROM, and its
+byte at offset 40 has been updated with the SD card interface used to
+boot, with its bit 5 set to `1` if it was loaded from offset 128KB
+from the SD Card, or `0` if was loaded from offset 8KB.
+
+The role of the SPL stage is to set up the CPU clocks to their nominal
+speeds and set up the main SDRAM memory in order to load into it the
+next boot stage: either the U-Boot bootloader or the Linux kernel, as
+these are too large to fit into the small SRAM memory.
+
+[1]: ../boot_rom
 
 --8<--
 includes/glossary.md