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: 13610aa9 ("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>