Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/external/vanetza-idf/vanetza/asn1/patches/system_header.patch

@@ -0,0 +1,22 @@
+diff --git a/vanetza/asn1/support/asn_system.h b/vanetza/asn1/support/asn_system.h
+index 51859cdd..c97a9ad5 100644
+--- a/vanetza/asn1/support/asn_system.h
++++ b/vanetza/asn1/support/asn_system.h
+@@ -125,7 +125,7 @@ typedef	unsigned int	uint32_t;
+ #endif /* __GNUC__ */
+ #endif	/* MIN */
+ 
+-#if __STDC_VERSION__ >= 199901L
++#if __STDC_VERSION__ >= 199901L || __cplusplus >= 201103L
+ #ifndef SIZE_MAX
+ #define SIZE_MAX   ((~((size_t)0)) >> 1)
+ #endif
+@@ -145,7 +145,7 @@ typedef	unsigned int	uint32_t;
+ #define RSSIZE_MAX   ((ssize_t)(RSIZE_MAX >> 1))
+ #endif
+ 
+-#if __STDC_VERSION__ >= 199901L
++#if __STDC_VERSION__ >= 199901L || __cplusplus >= 201103L
+ #define ASN_PRI_SIZE "zu"
+ #define ASN_PRI_SSIZE "zd"
+ #define ASN_PRIuMAX PRIuMAX