x86/boot: Add missing file header comments

There were some files with missing header comments. Since they are
included from both compressed and regular kernels, make note of that.
Also corrects a typo in the mem_avoid comments.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: kernel-hardening@lists.openwall.com
Cc: lasse.collin@tukaani.org
Link: http://lkml.kernel.org/r/1462825332-10505-3-git-send-email-keescook@chromium.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/compressed/kaslr.c

@@ -163,7 +163,7 @@ static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
  * memory ranges lead to really hard to debug boot failures.
  *
  * The initrd, cmdline, and boot_params are trivial to identify for
- * avoiding. The are MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, and
+ * avoiding. They are MEM_AVOID_INITRD, MEM_AVOID_CMDLINE, and
  * MEM_AVOID_BOOTPARAMS respectively below.
  *
  * What is not obvious how to avoid is the range of memory that is used

arch/x86/boot/early_serial_console.c

@@ -1,3 +1,7 @@
+/*
+ * Serial port routines for use during early boot reporting. This code is
+ * included from both the compressed kernel and the regular kernel.
+ */
 #include "boot.h"
 
 #define DEFAULT_SERIAL_PORT 0x3f8 /* ttyS0 */

arch/x86/mm/ident_map.c

@@ -1,3 +1,7 @@
+/*
+ * Helper routines for building identity mapping page tables. This is
+ * included by both the compressed kernel and the regular kernel.
+ */
 
 static void ident_pmd_init(unsigned long pmd_flag, pmd_t *pmd_page,
 			   unsigned long addr, unsigned long end)
@@ -10,6 +14,7 @@ static void ident_pmd_init(unsigned long pmd_flag, pmd_t *pmd_page,
 			set_pmd(pmd, __pmd(addr | pmd_flag));
 	}
 }
+
 static int ident_pud_init(struct x86_mapping_info *info, pud_t *pud_page,
 			  unsigned long addr, unsigned long end)
 {