Fixed undefined reference error when "make recover" in EON

In commaai/panda@fd23383
the linker flag -lgcc might not work on EON as it does not have the
correct library.

The fix was a workaround in sha.c such that we no longer need to import
function __aeabi_llsr from library, by replacing right shift operation
with const argument.

E.g.,
uint64_t a = b >> i // requires __aeabi_llsr from libgcc
uint64_t a = b >> 2 // does not require external library

Resolves: #522

panda/board/build.mk

@@ -55,7 +55,7 @@ obj/$(PROJ_NAME).bin: obj/$(STARTUP_FILE).o obj/main.$(PROJ_NAME).o
 
 
 obj/bootstub.$(PROJ_NAME).bin: obj/$(STARTUP_FILE).o obj/bootstub.$(PROJ_NAME).o obj/sha.$(PROJ_NAME).o obj/rsa.$(PROJ_NAME).o
-	$(CC) $(CFLAGS) -o obj/bootstub.$(PROJ_NAME).elf $^ -lgcc
+	$(CC) $(CFLAGS) -o obj/bootstub.$(PROJ_NAME).elf $^
 	$(OBJCOPY) -v -O binary obj/bootstub.$(PROJ_NAME).elf $@
 
 clean:

panda/crypto/sha.c

@@ -127,10 +127,36 @@ const uint8_t* SHA_final(SHA_CTX* ctx) {
     while ((ctx->count & 63) != 56) {
         SHA_update(ctx, (uint8_t*)"\0", 1);
     }
-    for (i = 0; i < 8; ++i) {
-        uint8_t tmp = (uint8_t) (cnt >> ((7 - i) * 8));
-        SHA_update(ctx, &tmp, 1);
-    }
+
+    /* Hack - right shift operator with non const argument requires
+     *        libgcc.a which is missing in EON
+     *        thus expanding for loop from 
+
+              for (i = 0; i < 8; ++i) {
+                  uint8_t tmp = (uint8_t) (cnt >> ((7 - i) * 8));
+                  SHA_update(ctx, &tmp, 1);
+              }
+
+              to
+     */
+
+    uint8_t tmp = 0;
+    tmp = (uint8_t) (cnt >> ((7 - 0) * 8));
+    SHA_update(ctx, &tmp, 1);
+    tmp = (uint8_t) (cnt >> ((7 - 1) * 8));
+    SHA_update(ctx, &tmp, 1);
+    tmp = (uint8_t) (cnt >> ((7 - 2) * 8));
+    SHA_update(ctx, &tmp, 1);
+    tmp = (uint8_t) (cnt >> ((7 - 3) * 8));
+    SHA_update(ctx, &tmp, 1);
+    tmp = (uint8_t) (cnt >> ((7 - 4) * 8));
+    SHA_update(ctx, &tmp, 1);
+    tmp = (uint8_t) (cnt >> ((7 - 5) * 8));
+    SHA_update(ctx, &tmp, 1);
+    tmp = (uint8_t) (cnt >> ((7 - 6) * 8));
+    SHA_update(ctx, &tmp, 1);
+    tmp = (uint8_t) (cnt >> ((7 - 7) * 8));
+    SHA_update(ctx, &tmp, 1);
 
     for (i = 0; i < 5; i++) {
         uint32_t tmp = ctx->state[i];