Makefile: fix the {c, e}tags/cscope build targets

Makefile: fix the {c, e}tags/cscope build targets

Commit 9e41403 (kbuild: change out-of-tree build)
changed the U-Boot build working directory to the output tree
for the out-of-tree builds.
This broke the {c,e}tags/cscope build targets as TAG_SUBDIRS variable
collected directories based on assumption that the build working
directory is the U-Boot source tree directory.

Fix the {c,e}tags/cscope build targets by adding the $(srctree) prefix.
Also, remove the $(obj) prefix from the etags build target to finish
the $(obj) prefix removal started by the same commit.

Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>