km_arm: use filesize for erase in update command

We used to have an arbitrary value, which can be a problem if we have a
u-boot image that is bigger than this value.

This patch is dependant on the whole km/arm series and will be included
in the v3 of the series if there is one.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Gerlando Falauto <gerlando.falauto@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>

include/configs/km/km_arm.h

@@ -241,7 +241,7 @@ int get_scl(void);
 
 #define	CONFIG_KM_UPDATE_UBOOT						\
 	"update="							\
-		"spi on;sf probe 0;sf erase 0 50000;"			\
+		"spi on;sf probe 0;sf erase 0 +${filesize};"		\
 		"sf write ${load_addr_r} 0 ${filesize};"		\
 		"spi off\0"