Lock memchr version to support edition 2018

memchr 2.5.0 is the last version that supports edition 2018. lock the
version in Cargo.toml

Bug: 297895455
Change-Id: If51fa5ba9e284ade71b49013f5e8980bc19539c8

hal/Cargo.toml

diff --git a/hal/Cargo.toml b/hal/Cargo.toml
index 82d74bc..18b0798 100644
--- a/hal/Cargo.toml
+++ b/hal/Cargo.toml
@@ -4,6 +4,9 @@
 edition = "2018"
 
 [dependencies]
-core2 = { version = "0.3", default-features = false }
+# Lock memchr to 2.5.0, the last version for 2018. Set default features to false
+# to disable std.
+memchr = { version = "=2.5.0", default-features = false }
+core2 = { version = "=0.3.3", default-features = false }
 kernel          = { path = "../../tock/kernel" }
 matcha_config   = { path = "../config" }