treewide: Kbuild: Remove unused `shell date` commands from macro defines

This macro define changes every build due to the nature of datetime,
causing the build-system to needlessly recompile all source files every
time even though the macro is never referenced (grep for
BUILD_TIMESTAMP).

[Pavel Dubrova]
Applied to all existing BUILD_TIMESTAMPs.

Signed-off-by: Marijn Suijten <marijns95@gmail.com>
Signed-off-by: Pavel Dubrova <pashadubrova@gmail.com>
Change-Id: I5816a8a54d84b477f892a721bfd3957b28394ea2