env: allow to copy value from default environment into a buffer

env_get_default suffers from a particular issue int that it can only
return a value truncated to gd->env_buf (32) characters. This may be
enough for most variables but it isn't for others, so let's allow users
to provide a preallocated buffer to copy the value into instead,
allowing for more control, though it'll still be truncated if the value
size is bigger than the preallocated buffer.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>

env/common.c

@@ -359,6 +359,14 @@ char *env_get_default(const char *name)
 	return NULL;
 }
 
+/*
+ * Look up the variable from the default environment and store its value in buf
+ */
+int env_get_default_into(const char *name, char *buf, unsigned int len)
+{
+	return env_get_from_linear(default_environment, name, buf, len);
+}
+
 void env_set_default(const char *s, int flags)
 {
 	if (s) {

include/env.h

@@ -356,6 +356,16 @@ int env_import_redund(const char *buf1, int buf1_read_fail,
  */
 char *env_get_default(const char *name);
 
+/**
+ * env_get_default_into() - Look up a variable from the default environment and
+ * copy its value in buf.
+ *
+ * @name: Variable to look up
+ * Return: actual length of the variable value excluding the terminating
+ *	NULL-byte, or -1 if the variable is not found
+ */
+int env_get_default_into(const char *name, char *buf, unsigned int len);
+
 /* [re]set to the default environment */
 void env_set_default(const char *s, int flags);