manual: use current date/time for 'generated on'

Currently, the manual uses the last modification date and time of
manual.txt in the generated manual.

This is confusing, especially for long-checked-out repositories where
the top-level manual.txt has not changed since the check out. Moreover,
the manual explicitly states 'generated on', which is confusing at best.

Use the current date and time instead.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/manual.txt

@@ -8,8 +8,8 @@ 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}
+Buildroot {sys:echo $\{BR2_VERSION%%-git*\}} manual generated on {localdate}
+{localtime} from git revision {sys:git rev-parse --short HEAD}
 
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