Improve comments in Makefile

Makefile

@@ -1,4 +1,4 @@
-#This is a simple wrapper which hides cmake (for convenience, and from non-expert end users).
+# This is a simple wrapper which hides cmake (for convenience, and from non-expert end users).
 #
 # It supports the targets:
 #   'make'           - builds everything (all libaries/executables)
@@ -15,12 +15,14 @@
 #
 #   'make BUILD_TYPE=debug VERBOSE=1'
 
-#Default build type
-# Possible values:
-#    release_pgo	#Perform a 2-stage build with profile-guided compiler optimization
-#    release		#Build with compiler optimization
-#    debug			#Build with debug info and no compiler optimization
-#    strict			#Build VPR with warnings treated as errors
+# Build type
+# Possible values (not case sensitive):
+#    release            #Build with compiler optimization (Default)
+#    RelWithDebInfo     #Build with debug info and compiler optimizations
+#    debug              #Build with debug info and no compiler optimization
+# Possible suffixes:
+#    _pgo               #Perform a 2-stage build with profile-guided compiler optimization
+#    _strict            #Build VPR with warnings treated as errors
 BUILD_TYPE ?= release
 
 #Debugging verbosity enable