fix: Fix MD5 option handling

In order to avoid breaking changes, the non-prefixed MD5_ENABLED option
has to be handled in a special way instead of using the add_option()
function.

Closes #112

CMakeLists.txt

@@ -52,7 +52,9 @@ if (DEFINED SERIAL_FLASHER_INTERFACE_UART OR CONFIG_SERIAL_FLASHER_INTERFACE_UAR
     list(APPEND defs
         SERIAL_FLASHER_INTERFACE_UART
     )
-    add_option(MD5_ENABLED false)
+    if (DEFINED MD5_ENABLED OR CONFIG_SERIAL_FLASHER_MD5_ENABLED)
+        list(APPEND defs MD5_ENABLED=1)
+    endif()
 
 elseif(DEFINED SERIAL_FLASHER_INTERFACE_USB OR CONFIG_SERIAL_FLASHER_INTERFACE_USB STREQUAL "y")
     list(APPEND srcs
@@ -62,7 +64,9 @@ elseif(DEFINED SERIAL_FLASHER_INTERFACE_USB OR CONFIG_SERIAL_FLASHER_INTERFACE_U
     list(APPEND defs
         SERIAL_FLASHER_INTERFACE_USB
     )
-    add_option(MD5_ENABLED false)
+    if (DEFINED MD5_ENABLED OR CONFIG_SERIAL_FLASHER_MD5_ENABLED)
+        list(APPEND defs MD5_ENABLED=1)
+    endif()
 
 elseif(DEFINED SERIAL_FLASHER_INTERFACE_SPI OR CONFIG_SERIAL_FLASHER_INTERFACE_SPI STREQUAL "y")
     list(APPEND srcs