Rename ethos-u to ethos_u.

Addresses http://b/168824958

tensorflow/lite/micro/kernels/ethos_u/README.md

@@ -1,6 +1,8 @@
+<!-- mdformat off(b/169948621#comment2) -->
+
 # Info
 
-To use Ethos-U kernel add TAGS="ethos-u" to the make line. A tflite file
+To use Ethos-U kernel add CO_PROCESSOR=ethos_u to the make line. A tflite file
 compiled by ARM's offline tool Vela is required for it to work. Armclang 6.14 is
 required as compiler as well.
 
@@ -39,6 +41,6 @@ makefile.
 Compile a binary with Ethos-U kernel.
 
 ```
-make -f tensorflow/lite/micro/tools/make/Makefile network_tester_test TAGS="ethos-u" \
+make -f tensorflow/lite/micro/tools/make/Makefile network_tester_test CO_PROCESSOR=ethos_u \
 TARGET=<ethos_u_enabled_target> NETWORK_MODEL=<ethos_u_enabled_tflite>
 ```

tensorflow/lite/micro/tools/make/Makefile

@@ -103,16 +103,17 @@ MICROLITE_LIBS := -lm
 #   1. Convert to uppercase (TARGET=xtensa -> -DXTENSA)
 #   2. (Temporarily) Replace dash with underscore (OPTIMIZED_KERNEL_DIR=cmsis-nn -> -DCMSIS_NN)
 #
-# Transformation 2 is needed because CMSIS-NN and ETHOS-U are not a valid macro names.
-
+# Transformation 2 is needed because CMSIS-NN is not a valid macro names.
 ADDITIONAL_DEFINES := -D$(shell echo $(TARGET) | tr [a-z] [A-Z])
 
 ifneq ($(OPTIMIZED_KERNEL_DIR),)
+  # TODO(b/168824958): remove dash->underscore transformation once the cmsis-nn
+  # directory has been renamed.
   ADDITIONAL_DEFINES += -D$(shell echo $(OPTIMIZED_KERNEL_DIR) | tr [a-z] [A-Z] | tr - _)
 endif
 
 ifneq ($(CO_PROCESSOR),)
-  ADDITIONAL_DEFINES += -D$(shell echo $(CO_PROCESSOR) | tr [a-z] [A-Z] | tr - _)
+  ADDITIONAL_DEFINES += -D$(shell echo $(CO_PROCESSOR) | tr [a-z] [A-Z])
 endif
 
 OPTIMIZATION_LEVEL := -O3