spi_flash: add generic SPI NOR flash layer (Phase 3)

Transport-agnostic (xfer callback) layer with a JEDEC-ID chip database
and detect/read/erase_sector/program_page/program/verify. Standard
opcode set, 3- and 4-byte addressing (4-byte command opcodes for parts
over 16 MB). Seeded with the KCU105's MT25QU256 plus common
Winbond/Macronix/ISSI/Micron parts.

detect + read validated on the KCU105 over the proxy. erase/program are
implemented but not yet hardware-tested (destructive on a config flash).

modules/spi_flash/CMakeLists.txt

@@ -0,0 +1,7 @@
+set(SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
+
+file(GLOB_RECURSE ALL_SOURCES "*.c")
+
+include_directories(${CMAKE_CURRENT_SOURCE_DIR}/..)
+
+add_library(spi_flash ${ALL_SOURCES})