smbios: Use integer signature instead of string signature

Change the smbios structure to use a 4 byte u32 signature field
instead of a 4 byte character string field.  In practice, this allows
the compiler to place the signature in the initialize code segment and
thus makes it less likely the signature would be found in the
f-segment.  (If the smbios signature is found in the f-segment it can
confuse some table scans.)

Reviewed-by: Bruce Rogers <brogers@suse.com>
Tested-by: Bruce Rogers <brogers@suse.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/fw/biostables.c b/src/fw/biostables.c
index 50a891b..450aca2 100644
--- a/src/fw/biostables.c
+++ b/src/fw/biostables.c
@@ -271,7 +271,7 @@
     if (SMBiosAddr)
         return;
     struct smbios_entry_point *p = pos;
-    if (memcmp(p->anchor_string, "_SM_", 4))
+    if (p->signature != SMBIOS_SIGNATURE)
         return;
     if (checksum(pos, 0x10) != 0)
         return;
diff --git a/src/fw/smbios.c b/src/fw/smbios.c
index dba0541..f3b5ad9 100644
--- a/src/fw/smbios.c
+++ b/src/fw/smbios.c
@@ -37,7 +37,7 @@
 
     struct smbios_entry_point ep;
     memset(&ep, 0, sizeof(ep));
-    memcpy(ep.anchor_string, "_SM_", 4);
+    ep.signature = SMBIOS_SIGNATURE;
     ep.length = 0x1f;
     ep.smbios_major_version = 2;
     ep.smbios_minor_version = 4;
diff --git a/src/std/smbios.h b/src/std/smbios.h
index 0513716..4ccf2ea 100644
--- a/src/std/smbios.h
+++ b/src/std/smbios.h
@@ -3,11 +3,13 @@
 
 #include "types.h" // u32
 
+#define SMBIOS_SIGNATURE 0x5f4d535f // "_SM_"
+
 /* SMBIOS entry point -- must be written to a 16-bit aligned address
    between 0xf0000 and 0xfffff.
  */
 struct smbios_entry_point {
-    char anchor_string[4];
+    u32 signature;
     u8 checksum;
     u8 length;
     u8 smbios_major_version;