What does the assembly command bl mean?
bl = branch and link. It is the way to call functions in powerpc family. The return address is stored in the LR (Link Register). The function returns using blr (branch to link register). You need PPC manual for your CPU to understand these. I like the IBM ones (as opposed to Motorola ones) more useful. If you are using PPC405GP search for the manual using Google. If you cannot find it let me know I'll send you the link to PDF file.
What does the line bl board_init_f do?
It calls board_init_f function. This is part of starup code. board_init_f() does initializations while your code is executing from flash. For example, board_init_f() calls board_pre_init(). Read the source file containing board_init_f()
Regards, Tolunay