Serial#Find method for Arduino, implemented in Mbed

Adoption of https://www.arduino.cc/en/Serial/Find, implemented without use of interrupts or buffers. Note that it keeps the thread busy, so if you're running this in the background, better use Serial#attach with a buffer.

Revision:
0:6228e490f311
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/.hgignore	Fri Nov 10 21:11:04 2017 +0900
@@ -0,0 +1,5 @@
+BUILD/
+mbed-os/
+.mbed
+mbed_settings.*
+.git/