efi_loader: use ':' as separator for setenv -i

setenv -e -i <address>,<filesize> can be used to set a UEFI variable
from memory.

For separating an address and a size we use ':' in most commands.
Let's do the same for setenv -e -i.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>