zephyr: Disable and deprecate flash erase

This deprecates the flash erase Kconfig for zephyr, if this action
is required then the board should be flashed using west with the
`--erase` argument supplied instead.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index e89a0b6..a93be08 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -481,8 +481,8 @@
 endif # BOOT_USB_DFU_GPIO
 
 config ZEPHYR_TRY_MASS_ERASE
-	bool "Try to mass erase flash when flashing MCUboot image"
-	default y
+	bool "Try to mass erase flash when flashing MCUboot image (DEPRECATED)"
+	select DEPRECATED
 	help
 	  If y, attempt to configure the Zephyr build system's "flash"
 	  target to mass-erase the flash device before flashing the
@@ -491,6 +491,9 @@
 
 	  This is not available for all targets.
 
+	  This option has been deprecated, to perform a mass erase when
+	  flashing a board, `west flash --erase` should be used instead.
+
 config BOOT_USE_BENCH
         bool "Enable benchmark code"
         default n