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/Kconfig b/cmd/Kconfig
index 3b3722f..a2b9e19 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1671,6 +1671,12 @@
 	     sound init   - set up sound system
 	     sound play   - play a sound
 
+config CMD_SYSBOOT
+	bool "sysboot"
+	select MENU
+	help
+	  Boot image via local extlinux.conf file
+
 config CMD_QFW
 	bool "qfw"
 	select QFW