BUILD: ssl_utils: fix build on gcc versions before 8

BUILD: ssl_utils: fix build on gcc versions before 8

Commit 960fb74ca ("MEDIUM: ssl: {ca,crt}-ignore-err can now use error
constant name") provided a very convenient way to initialize only desired
macros. Unfortunately with gcc versions older than 8, it breaks with:

  src/ssl_utils.c:473:12: error: initializer element is not constant

because it seems that the compiler cannot resolve strings to constants
at build time.

This patch takes a different approach, it stores the value of the macro
as a string and this string is converted to integer at boot time. This
way it works everywhere.