targets: add implementation for Tillitis TKey device (#4631)

* initial implementation for Tillitis TKey device
* add UART implementation for TKey
* add Pin interface implementation for TKey touch sensor
* add RNG interface implementation for TKey
* add helpful machine package functions to return identifiers such as name and version for TKey
* use built-in timer for sleep timing on TKey
* modify UART implementation for TKey to implement Serialer interface
* implement BLAKE2s ROM function call for TKey device
* handle abort by triggering TKey device fault using illegal instruction to halt CPU
* simplify TKey implementation by inheriting from existing riscv32 target
* return error for trying to configure invalid baudrates on UART
* add tkey to builder test
* be very specific for features passed to LLVM for specific config in use for TKey
* handle feedback items from TKey device code review

Signed-off-by: deadprogram <ron@hybridgroup.com>

targets/tkey.ld

@@ -0,0 +1,11 @@
+
+MEMORY
+{
+    RAM (rwx)       : ORIGIN = 0x40000000, LENGTH = 0x20000 /* 128 KB */
+}
+
+REGION_ALIAS("FLASH_TEXT", RAM);
+
+_stack_size = 2K;
+
+INCLUDE "targets/riscv.ld"