imgtool: fix boot_magic when -e big and max-align > 8

Currently if max-align > 8, magic_boot only works in little endian

For example :
With max-align = 16 and endian = big, boot_magic starts with 0x10 0x00,
but it should be 0x00 0x10

Signed-off-by: Raphael Dupont <raphael7dup@gmail.com>