HAL_ChibiOS: moved app_descriptor to start of flash

this prevents the bootloader from finding it's own image as a false
posititve in searching for the app_descriptor

libraries/AP_HAL_ChibiOS/hwdef/common/common.ld

@@ -70,6 +70,9 @@ SECTIONS
 
     .text : ALIGN(4) SUBALIGN(4)
     {
+        /* we want app_descriptor near the start of flash so a false
+           positive isn't found by the bootloader (eg. ROMFS) */
+        KEEP(*(.app_descriptor));
         *(.text)
         *(.text.*)
         *(.rodata)