apps/boot - Remove .OVERWRITE flag
apps/boot - Remove .OVERWRITE flag

apps/boot specified the .OVERWRITE flag when `BOOT_SERIAL` was set.
This flag should not be here, as the serial boot loader requires all the
normal dependencies (boot/bootutil, kernel/os, and sys/console/stub).
These are required to satisfy other dependencies' API requirements.

andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/apache-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,andrzej-kaczmarek/apache-mynewt-core,andrzej-kaczmarek/incubator-mynewt-core