Document the hw.physmem kernel environment variable.

Reviewed by:	alfred

sys/boot/common/loader.8

@@ -404,6 +404,13 @@ MIB variable.
 Other variables are used to override kernel tunable parameters.
 The following tunables are available:
 .Bl -tag -width Va
+.It Va hw.physmem
+Limit the amount of phyiscal memory the system will use.
+By default the size is in bytes, but the k, K, m, M, g and G suffixes
+are also accepted and indicate kilobytes, megabytes and gigabytes
+respectively.
+An invalid suffix will result in the variable being ignored by the
+kernel.
 .It Va kern.maxusers
 Set the size of a number of statically allocated system tables; see
 .Xr tuning 7

sys/boot/forth/loader.conf

@@ -71,6 +71,7 @@ module_path="/boot/kernel;/boot/modules;/modules"	# Set the module search path
 ###  Kernel tunables  ########################################
 ##############################################################
 
+#hw.physmem="1G"		# Limit phyiscal memory. See loader(8)
 #kern.maxusers="32"		# Set size of various static tables
 #kern.ipc.nmbclusters=""	# Set the number of mbuf clusters
 #kern.vm.kmem.size=""		# Sets the size of kernel memory (bytes)