scripts/gdb: change kernel config dumping method

MAGIC_START("IKCFG_ST") and MAGIC_END("IKCFG_ED") are moved out
from the kernel_config_data variable.

Thus, we parse kernel_config_data directly instead of considering
offset of MAGIC_START and MAGIC_END.

Fixes: 13610aa908dc ("kernel/configs: use .incbin directive to embed config_data.gz")
Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/gdb: add kernel config dumping command

lx-configdump <file> dumps the contents of the gzipped .config to a text
file when the config is included in the kernel with CONFIG_IKCONFIG. By
default, the file written is called config.txt, but it can be any user
supplied filename as well. If the kernel config is in a module
(configs.ko), then it can be loaded along with symbols for the module
loaded with 'lx-symbols' and then this command will still work.

Obviously if you have the whole vmlinux then this can also be achieved
with scripts/extract-ikconfig, but this gdb script can be useful to
confirm that the memory contents of the config in memory and the vmlinux
contents on disk match what is expected.

[swboyd@chromium.org: v2]
Link: http://lkml.kernel.org/r/20190329220844.38234-3-swboyd@chromium.org
Link: http://lkml.kernel.org/r/20190325184522.260535-3-swboyd@chromium.org
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Nikolay Borisov <n.borisov.lkml@gmail.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jackie Liu <liuyun01@kylinos.cn>
Cc: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>