security/vboot: Add Kconfig symbol to set hashing block size

Generally, this size probably doesn't matter very much, but in the
case of picasso's psp_verstage, the hash is being calculated by
hardware using relatively expensive system calls.  By increasing the
block size, we can save roughly 140ms of boot and resume time.

TEST=Build & boot see that boot time has decreased.
BRANCH=Zork
BUG=b:169217270 - Zork: SHA calculation in vboot takes too long

Signed-off-by: Martin Roth <martinroth@chromium.org>
Change-Id: I68eecbbdfadcbf14288dc6e849397724fb66e0b2
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46901
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Kangheui Won <khwon@chromium.org>

src/security/vboot/Kconfig

@@ -369,6 +369,17 @@ config VBOOT_KEYBLOCK_PREAMBLE_FLAGS
 	hex "Keyblock preamble flags"
 	default 0x0
 
+config VBOOT_HASH_BLOCK_SIZE
+	hex
+	default 0x400
+	help
+	  Set the default hash size.  Generally 1k is reasonable, but in some
+	  cases it may improve hashing speed to increase the size.
+
+	  Note that this buffer is allocated in the stack.  Although the
+	  build should fail if the stack size is exceeded, it's something to
+	  be aware of when changing the size.
+
 endmenu # Keys
 endif # VBOOT
 endmenu # Verified Boot (vboot)