doc/util/ifdtool: Update instructions

- Add step for building ifdtool (might not be obvious)
- Remove "./ifdtool COREBOOT_NAME" because it does nothing
- Add a small comment explaining what the -d and -x args do.

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I868ea8918a1566cfade3bc161117f2ca8dfed31d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/85235
Reviewed-by: Nicholas Chin <nic.c3.14@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

Documentation/util/ifdtool/binary_extraction.md

@@ -9,9 +9,9 @@ Now, execute the following commands to extract the binaries from a ROM image.
 **Note:** Make sure you are in the root coreboot directory.
 
     cd /path/to/coreboot/util/ifdtool
-    ./ifdtool COREBOOT_IMAGE
-    ./ifdtool -d COREBOOT_IMAGE
-    ./ifdtool -x COREBOOT_IMAGE
+    make
+    ./ifdtool -d COREBOOT_IMAGE # dump flash descriptor to stdout
+    ./ifdtool -x COREBOOT_IMAGE # extract IFD binaries into files (e.g. ME, IFD, GBE...)
 
 In the above steps, COREBOOT_IMAGE is the name of the ROM image to extract the
 binaries from, including the file path (ex. /build/coreboot.rom).