elf: fix cache flushing in 'bootelf -p' command

elf: fix cache flushing in 'bootelf -p' command

Currently there are two problems in 'bootelf -p' (load elf by segments)
command:
- bss section is not flushed, so booted elf can have non zero values
  in bss;
- at least on ARM there are 'CACHE: Misaligned operation at
  range...' warnings

Use p_memsz instead of p_filesz during cache flushing for elf segment.
p_filesz doesn't include zero initialized memory (e.g. bss section),
which also should be flushed.

Align these cache flushes to line boundaries.

Signed-off-by: Kurban Mallachiev <mallachiev@ispras.ru>