u-boot.elf: allow overriding entry symbol

LD gives the following warning when trying to process u-boot-elf.o
warning: cannot find entry symbol __start; defaulting to 0000000080010000
According to gnu-libc the entry symbol for mips is __start and not _start:
https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/mips/dl-machine.h;h=ed47513ccc1d23d23d32ee640053d2f351f3990b;hb=HEAD#l258



Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>