Fix: address Arm compiler licensing issue.

Fix for issue T19: https://issues.trustedfirmware.org/T19

Change details:
CompilerArmClangCommon.cmake:
  - Removed warning issued by cmake about incorrect licensing settings.

tfm_sw_requirement.md
  - Fixed incorrect Keil specific environment variable settings.
  - Copied licensing related warning to multiple places to make it more
    visible.
  - Some small improvements to help readability.

Change-Id: I6656ad0e9d9d668b2d2b70b18c19783b7cac03bf
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
diff --git a/cmake/Common/CompilerArmClangCommon.cmake b/cmake/Common/CompilerArmClangCommon.cmake
index 9b69f8a..53f143a 100644
--- a/cmake/Common/CompilerArmClangCommon.cmake
+++ b/cmake/Common/CompilerArmClangCommon.cmake
@@ -1,5 +1,5 @@
 #-------------------------------------------------------------------------------
-# Copyright (c) 2017-2018, Arm Limited. All rights reserved.
+# Copyright (c) 2017-2019, Arm Limited. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -21,11 +21,6 @@
 		message(FATAL_ERROR "ARMClang version (ARMCLANG_VER=${ARMCLANG_VER}) does not match ${MY_VERSION}")
 	endif()
 
-	#Emit warning if needed environment variables are not set.
-	if(NOT DEFINED ENV{ARM_TOOL_VARIANT} OR NOT DEFINED ENV{ARM_PRODUCT_PATH})
-		message(WARNING "ARM_TOOL_VARIANT or ARM_PRODUCT_PATH environment variables are not set!")
-	endif()
-
 	if (NOT DEFINED ARM_CPU_ARCHITECTURE AND NOT DEFINED ARM_CPU_TYPE)
 		message(FATAL_ERROR "ARM_CPU_TYPE and ARM_CPU_ARCHITECTURE is not defined! Please include the CPU specific config file before this one.")
 	endif()