boot: pxe: Use bootm_...() functions where possible

Rather than building a command line for each operation, use the
functions provided by the bootm API.

Make sure that the bootm functions are available if pxe_utils is used.

Since SYS_BOOTM_LEN is not present for the tools-only build, adjust the
code to handle that.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
2 files changed