x86: use read wrappers in kernel loading

Replace the unsafe read-loops in the x86 kernel image loading
functions with our safe read_file() and read_in_full() wrappers.
This should fix random fails in kernel image loading, especially
from pipes and sockets.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>