message: Add comment explaining prerequisites to run for target hexagon.  Add check that HEXAGON_SDK_PREFIX is set when TARGET=hexagon is specified.
diff: # Settings for Hexagon toolchain.
  +# REQUIRED:
  +# - Hexagon SDK 3.5 Toolkit (for hexagon-clang++, hexagon-sim).
  +# - HEXAGON_SDK_PREFIX environment variable must be set to location of
  +# Hexagon_SDK/<version>/ on your machine.
  +#
  +# Unlike other targets, there is not currently a way to automatically download
  +# the Hexagon SDK. For this reason, users are required to manually download
  +# and configure the SDK.
  ifeq ($(TARGET), hexagon)
  TARGET_ARCH := hexagon
  @@ -51,6 +59,10 @@ ifeq ($(TARGET), hexagon)
  LDFLAGS += \
  -Wl,--gc-sections -lhexagon
  + ifndef HEXAGON_SDK_PREFIX
  + $(error HEXAGON_SDK_PREFIX is undefined)
  + endif
  +
  INCLUDES += \
  -I$(HEXAGON_SDK_PREFIX)/libs/common/qurt/computev66/include/posix \
  -I$(HEXAGON_SDK_PREFIX)/libs/common/qurt/computev66/include/qurt
  