[sw/ottf] Rename test_rom_ext to ottf_start

Previously the boot sequence for chip level tests looked like:

chip reset --> test_rom --> test_rom_ext --> ottf --> test_main()

In the above sequence the `test_rom_ext` was really just an ASM file
that performed initializations prior to jumping to the OTTF `main()`.
However, as we refactor the OTTF to enable running chip-level tests at
various boot stages (e.g., ROM_EXT and BL0, see #10498), it makes more
sense to model the combined OTTF and test bundle as its own boot stage
that can be run in inplace of the ROM_EXT or BL0.

Therefore, this commit renames/consolidates the test_rom_ext into a
component of the OTTF, called the `ottf_start`. The `ottf_start` is kept
as a separate static library from the `ottf` to enable running tests
that do not use the OTTF, i.e. the crt_test (which tests the
functionality of the `ottf_start` library), and the "Hello World"
example programs. However, when test images are built, both the
`ottf_start` and `ottf` are linked with the test library itself, to
create one cohesive "test boot stage".

The new boot sequence for a test will look like:

chip reset --> test_rom --> ottf --> test_main(),

where OTTF = (ottf_start --> OTTF).

This partially addresses a task in #10498.

Signed-off-by: Timothy Trippel <ttrippel@google.com>

sw/device/silicon_creator/mask_rom/meson.build

diff --git a/sw/device/silicon_creator/mask_rom/meson.build b/sw/device/silicon_creator/mask_rom/meson.build
index 02025b3..d0ef04e 100644
--- a/sw/device/silicon_creator/mask_rom/meson.build
+++ b/sw/device/silicon_creator/mask_rom/meson.build
@@ -4,7 +4,7 @@
 
 # Mask ROM Linker Parameters
 #
-# See `sw/device/lib/testing/test_rom_ext/test_rom_ext.ld` for additional info
+# See `sw/device/lib/testing/test_framework/ottf.ld` for additional info
 # about these parameters.
 
 rom_linkfile = files(['mask_rom.ld'])