[M] Change in coreboot[main]: lib/string: Add strncat() and strcat() functions

[Yu-Ping Wu (Code Review)]

Attention is currently required from: Jakub Czapiga, Julius Werner.

Yu-Ping Wu has posted comments on this change by Yu-Ping Wu. ( https://review.coreboot.org/c/coreboot/+/83765?usp=email )

Change subject: lib/string: Add strncat() and strcat() functions
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Patch Set 4:

(1 comment)

File src/lib/string.c:

https://review.coreboot.org/c/coreboot/+/83765/comment/ae9fa85d_67e9e060?usp... :
PS3, Line 107: 	strncpy(dst + strlen(dst), src, count);
> This isn't really a great implementation (walks the string twice). Better would be something like: […]

Does it walk the string twice? `strlen(dst)` walks `dst` once, and `strncpy` walks `src` once. BTW libpayload's implementation really walks `src` twice.

However I just found 2 problems in this implementation:

1. `strncpy` may not append a null byte when `count` is small.
2. Unlike `strncpy`, `dst` doesn't need to be padded with zeros (up to `count` bytes).

Therefore I decided to not use `strncpy` here. Also modified the test to catch the problem #1 above.