manual: add manual generation date/git revision in the manual text

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

docs/manual/manual.txt

@@ -8,6 +8,9 @@ The Buildroot user manual
 Buildroot usage and documentation by Thomas Petazzoni. Contributions
 from Karsten Kruse, Ned Ludd, Martin Herren and others.
 
+Buildroot {sys:echo $\{BR2_VERSION%%-git*\}} manual generated on {docdate}
+{doctime} from git revision {sys:git rev-parse --short HEAD}
+
 image::logo.png[]
 
 :leveloffset: 1