libflashprog: Introduce new probing API flashprog_flash_probe_chip()

This new probing API acts about the same as the old one, except that
it accepts a `struct flashprog_chip *` handle instead of a chip name.
Under the hood, however, it uses our new bus probing directly.

We introduce a new function, flashprog_flash_prepare_context(), that
creates a flash context from a probed chip, and the bus it was found
on. This will be the single place that creates flash contexts in the
future.

Change-Id: I1a344aedf573d92f8f297e6eed8daadb511ebc10
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/484
diff --git a/libflashprog.map b/libflashprog.map
index 033d5f8..42ebc86 100644
--- a/libflashprog.map
+++ b/libflashprog.map
@@ -18,6 +18,7 @@
     flashprog_flash_erase;
     flashprog_flash_getsize;
     flashprog_flash_probe;
+    flashprog_flash_probe_chip;
     flashprog_flash_release;
     flashprog_image_read;
     flashprog_image_verify;