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>
diff --git a/boot/Makefile b/boot/Makefile
index 34bac26..f1e4444 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -10,7 +10,7 @@
 obj-$(CONFIG_CMD_BOOTZ) += bootm.o bootm_os.o
 obj-$(CONFIG_CMD_BOOTI) += bootm.o bootm_os.o
 
-obj-$(CONFIG_PXE_UTILS) += pxe_utils.o
+obj-$(CONFIG_PXE_UTILS) += bootm.o pxe_utils.o
 
 endif