Improve SDRAM initialization

[cepetr]

This issue suggests a minor improvement to the SDRAM initialization code:

1. sdram_init() function should reside in its own .c file and not be mixed with the ST BSP SDRAM driver code.
2. sdram_init() should be called from system_init() or preferably from the SystemInit() function, rather than directly from the boardloader’s main() function.

Although this function is used only on one of the discovery kits, it introduces an unnecessary dependency and adds unnecessary complexity to the code.