km_arm: use filesize for erase in update command

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>