arm: boot0 hook: move boot0 hook before '_start'

arm: boot0 hook: move boot0 hook before '_start'

The boot0 hook on ARM does not insert its payload before the vector
table. This is both a mismatch with thec comment above it and
contradict usage of the boot0 hook on ARM64.

To fix this (and unify the semantics for ARM and ARM64), we change the
boot0-hook semantics on ARM to match those on ARM64:
  (1) if a boot0-hook is present it is inserted at the start of
      the image
  (2) if a boot0-hook is present, emitting the ARM vector table
      (and the _start) symbol are suppressed in vectors.S and
      the boot0-hook has full control over where and when it
      wants to emit these

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>