build: make kernel entry symbol a config option

This might change for different ports, so make it configurable.

Signed-off-by: Gustavo Lima Chaves <gustavo.lima.chaves@intel.com>

Makefile

@@ -849,7 +849,7 @@ quiet_cmd_create-lnk = LINK    $@
 	echo "$(LINKFLAGPREFIX)-Map=$(KERNEL_NAME).map"; 			\
 	echo "-L $(objtree)/include/generated";					\
 	echo "-u _OffsetAbsSyms -u _ConfigAbsSyms"; 				\
-	echo "-e __start"; 						 	\
+	echo "-e $(CONFIG_KERNEL_ENTRY)";					\
 	echo "$(LINKFLAGPREFIX)--start-group";					\
 	echo "$(LINKFLAGPREFIX)--whole-archive";				\
 	echo "libapplication.a";						\

misc/Kconfig

@@ -98,6 +98,12 @@ config LINK_WHOLE_ARCHIVE
 	This options allows linking external libraries with the
 	--whole-archive option to keep all symbols.
 
+config KERNEL_ENTRY
+	string "Kernel entry symbol"
+	default "__start"
+	help
+	Code entry symbol, to be set at linking phase.
+
 endmenu
 
 menu "Compiler Options"