[U-Boot] [RFC] initcall mechanism introduction

Hi all,

Linux Kernel has for a long time a well-optimized mechanism for for calling initiallisation code. Import the the same functionnality to U-Boot.

Evenif it will increase a few U-Boot (999 bytes) as show in PATCH 3/3 if we convert the NET_MULTI it will decrease of 2212 bytes at the end

Best Regards, J.