cmd: sysboot: Create a sysboot command dedicated file

Extract all sysboot command related code from pxe.c to new sysboot.c
Update Kconfig to insure that DISTRO_DEFAULT select new CMD_SYSBOOT
command.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/cmd/Makefile b/cmd/Makefile
index 149daec..e87c2f1 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -130,6 +130,7 @@
 obj-$(CONFIG_CMD_SPI) += spi.o
 obj-$(CONFIG_CMD_STRINGS) += strings.o
 obj-$(CONFIG_CMD_SMC) += smccc.o
+obj-$(CONFIG_CMD_SYSBOOT) += sysboot.o pxe_utils.o
 obj-$(CONFIG_CMD_TERMINAL) += terminal.o
 obj-$(CONFIG_CMD_TIME) += time.o
 obj-$(CONFIG_CMD_TRACE) += trace.o