Build SDK sources with SDK headers only

Expose the generated glyphs.h to the SDK include path and drop the APP
include directories from INCLUDES_PATH_SDK, so SDK sources build
independently of the app's headers. Apps must not rely on app headers being
visible to SDK sources (the bitcoin_app_base family was updated accordingly).

Avoid re-scanning the filesystem on every compile: the SDK include list is
computed once (:=) and the app header scan (HEADERS_APP) is memoized on first
use, while the app include list stays lazy to still capture late app additions.