fix: remove 'uboot'/'u-boot' from fail keywords — too broad

Xilinx program_flash often outputs informational lines containing
'uboot' (mini u-boot image file, U-Boot version banner) which are
not errors. The 'failed' keyword already catches 'U-Boot failed'
and 'uboot failed' in actual failure scenarios.

src/flash_programmer.py

@@ -38,7 +38,7 @@ ProgressCallback = Callable[[str, str], None] | None
 
 # Keywords in program_flash output that indicate failure even with rc=0
 _FAIL_KEYWORDS = [
-    "uboot", "u-boot", "failed", "cannot", "abort",
+    "failed", "cannot", "abort",
     "invalid", "unsupported", "timeout",
 ]