kbuild: redo fake deps at include/ksym/*.h

Commit 0e0345b7

 ("kbuild: redo fake deps at include/config/*.h")
simplified the Kconfig/fixdep interaction a lot.

For CONFIG_FOO_BAR_BAZ, Kconfig now touches include/config/FOO_BAR_BAZ
instead of the previous include/config/foo/bar/baz.h .

This commit simplifies the TRIM_UNUSED_KSYMS feature in a similar way:

  - delete .h suffix
  - delete tolower()
  - put everything in 1 directory

For EXPORT_SYMBOL(FOO_BAR_BAZ), scripts/adjust_autoksyms.sh now touches
include/ksym/FOO_BAR_BAZ instead of include/ksym/foo/bar/baz.h .

This is more precise, avoiding possibly unnecessary rebuilds.

  EXPORT_SYMBOL(FOO_BAR_BAZ)
  EXPORT_SYMBOL(_FOO_BAR_BAZ)
  EXPORT_SYMBOL(__FOO_BAR_BAZ)

were previously mapped to the same header, include/ksym/foo/bar/baz.h
but now are handled separately.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>