README: Introduce vendor-specific tools

Add detailed description about what the vendor-specific tools shall do,
and how to configure meson to specify the vendor-specific tools.

It also updates meson_options.txt to use updated psutils from
phosphor-power by default.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I17a508b675a754e76645d08d49547f669385813e
diff --git a/meson_options.txt b/meson_options.txt
index f4e84b4..027c92a 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -29,7 +29,7 @@
 #   get_version <some-psu-path>
 option('PSU_VERSION_UTIL',
        type: 'string',
-       value: '/usr/bin/psutils --getversion',
+       value: '/usr/bin/psutils --raw --get-version',
        description: 'The command and arguments to get PSU version')
 
 # The PSU_VERSION_COMPARE_UTIL specifies an executable that accepts the PSU
@@ -39,7 +39,7 @@
 #   get_latest_version 0001 0002 0003 # output 0003
 option('PSU_VERSION_COMPARE_UTIL',
        type: 'string',
-       value: '/usr/bin/psutils --compare',
+       value: '/usr/bin/psutils --raw --compare',
        description: 'The command and arguments to compare PSU versions')
 
 # The PSU update service