Fix board init code to respect the C runtime environment

board_init_f_mem() alters the C runtime environment's
stack it is actually already using. This is not a valid
behaviour within a C runtime environment.

Split board_init_f_mem into C functions which do not alter
their own stack and always behave properly with respect to
their C runtime environment.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Thomas Chou <thomas@wytron.com.tw>