x86, boot: Exclude early_serial_console.c if can't use it.

Removes early_serial_console.c code if we don't have the config option that
enables it (EARLY_PRINTK). When disabling this code, make early_serial_base a
constant 0 to allow the compiler to optimize away the code that checks for
early_serial_base.

Signed-off-by: Joe Millenbach <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Gokul Caushik <[e-mail]>
Reviewed-by: Josh Triplett <[e-mail]>
Signed-off-by: H. Peter Anvin <[e-mail]>

arch/x86/boot/compressed/early_serial_console.c

···
 #include "misc.h"
 
+#ifdef CONFIG_EARLY_PRINTK
+
 int early_serial_base;
 
 #include "../early_serial_console.c"
+
+#endif

arch/x86/boot/compressed/misc.h

···
 int cmdline_find_option_bool(const char *option);
 
 /* early_serial_console.c */
+#ifdef CONFIG_EARLY_PRINTK
+
 extern int early_serial_base;
 void console_init(void);
+
+#else
+
+static const int early_serial_base;
+static inline void console_init(void)
+{ }
+
+#endif
 
 #endif