update to target nrf52840

Revision:
0:ecd06432fb4b
diff -r 000000000000 -r ecd06432fb4b mbed.bld
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mbed.bld	Thu Aug 13 12:23:41 2020 +0000
@@ -0,0 +1,1 @@
+https://os.mbed.com/users/mbed_official/code/mbed/builds/65be27845400
\ No newline at end of file