usign: update to latest Git HEAD

This update fixes usign signature verification on files with certain
file sizes triggering a bug in the shipped SHA-512 implementation.

5a52b37 sha512: fix bad hardcoded constant in sha512_final()
3e6648b README: replace unicode character
716c3f2 README: add reference to OpenBSD signify
86d3668 README: provide reference for ed25519 algorithm
939ec35 usign: main.c: describe necessary arguments for -G

Ref: https://forum.openwrt.org/t/signature-check-failed/41945
Signed-off-by: Jo-Philipp Wich <[email protected]>

package/system/usign/Makefile

@@ -5,9 +5,9 @@ PKG_RELEASE:=1
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL=$(PROJECT_GIT)/project/usign.git
-PKG_SOURCE_DATE:=2015-07-04
-PKG_SOURCE_VERSION:=ef6419142a3b0fbcddcccf536e3c1880302c6f89
-PKG_MIRROR_HASH:=9499ed7e40889b364e446a428e185c40986b75087888bd7e1496542457a6dbaa
+PKG_SOURCE_DATE:=2019-08-06
+PKG_SOURCE_VERSION:=5a52b379902471cef495687547c7b568142f66d2
+PKG_MIRROR_HASH:=9779f6d6718a7f7cd3e28aa7feefc9b3f4b0c7a85cb58ff18afbeb6b4372177a
 CMAKE_INSTALL:=1
 PKG_CHECK_FORMAT_SECURITY:=1
 PKG_USE_MIPS16:=0