zephyr: Move code under 'boot' directory

The Zephyr build systems makes some assumptions about the directory
layout, and encounters problems if the necessary path contains "../..".
To help this, place the zephyr directory next to the bootutil directory
so that the Makefile can just refer to "../bootutil".  This keeps all of
the build artifacts under the proper top-level directory.

Makefile

@@ -1,5 +1,5 @@
 BOARD ?= qemu_x86
-CONF_FILE = zephyr/prj.conf
+CONF_FILE = boot/zephyr/prj.conf
-SOURCE_DIR = zephyr/src
+SOURCE_DIR = boot/zephyr
 
 include ${ZEPHYR_BASE}/Makefile.inc

boot/zephyr/Makefile

@@ -1,8 +1,8 @@
 subdir-ccflags-y += -I$(PROJECT)/boot/bootutil/include
-subdir-ccflags-y += -I$(PROJECT)/zephyr/include
+subdir-ccflags-y += -I$(PROJECT)/boot/zephyr/include
 
 obj-y += main.o
 obj-y += flash_map.o hal_flash.o os.o
 obj-y += keys.o
 
-obj-y += ../../boot/bootutil/src/
+obj-y += ../bootutil/src/