src/main: rename --override-boot-slot named argument
src/main: rename --override-boot-slot named argument

Use BOOTNAME instead of SLOTNAME as this better reflects the required
content of the argument. You must not give a slot name here (as
'rootfs.0') but instead the 'bootname' (i.e. the name the bootloader
identifies this slot with) of the slot, such as 'A' or 'system0'.

Signed-off-by: Enrico Jorns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,ejoerns/rauc,rauc/rauc,rauc/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc