lib: hashtable: Prevent recursive calling of callback functions

lib: hashtable: Prevent recursive calling of callback functions

In case there are two variables which each implement env callback
that performs env_set() on the other variable, the callbacks will
call each other recursively until the stack runs out. Prevent such
a recursion from happening.

Example which triggers this behavior:
static int on_foo(...) { env_set("bar", 0); ... }
static int on_bar(...) { env_set("foo", 0); ... }
U_BOOT_ENV_CALLBACK(foo, on_foo);
U_BOOT_ENV_CALLBACK(bar, on_bar);

Signed-off-by: Christoph Niedermaier <cniedermaier@dh-electronics.com>
Suggested-by: Marek Vasut <marex@denx.de>