cmd: printf: add helper functions from busybox

Import the following helper functions from Busybox-1.33.1 which are
required by printf.c:

  process_escape_sequence from libbb/process_escape_sequence.c,
  skip_whitespace from libbb/skip_whitespace.c,
  overlapping_strcpy  from libbb/safe_strncpy.c

  src-url: https://git.busybox.net/busybox/


  commit bcc5b0e6caca6c7602a6a41f "Bump version to 1.33.1"
  version: 1.33.1

Signed-off-by: Roland Gaudig <roland.gaudig@weidmueller.com>