Spell *Boot Guard* with a space for official spelling

See for example Intel document *Secure the Network Infrastructure –
Secure Boot Methodologies* [1].

Change all occurrences with the command below:

    $ git grep -l BootGuard | xargs sed -i 's/BootGuard/Boot Guard/g'

[1]: https://builders.intel.com/docs/networkbuilders/secure-the-network-infrastructure-secure-boot-methodologies.pdf

Change-Id: I69fb64b525fb4799bcb9d75624003c0d59b885b5
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/60136
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/lib/Kconfig.cbfs_verification

@@ -13,7 +13,7 @@ config CBFS_VERIFICATION
 	  file as it gets loaded by chaining it to a trust anchor that is
 	  embedded in the bootblock. This only makes sense if you use some
 	  out-of-band mechanism to guarantee the integrity of the bootblock
-	  itself, such as Intel BootGuard or flash write-protection.
+	  itself, such as Intel Boot Guard or flash write-protection.
 
 	  If a CBFS image was created with this option enabled, cbfstool will
 	  automatically update the hash embedded in the bootblock whenever it