lib: rsa: consider CONFIG_SPL_RSA

CONFIG_SPL_RSA is meant to control if lib/rsa/* is used for SPL. Adjust
lib/Makefile to consider this setting.

This was correctly setup with commit 51c14cd128f4 ("verified-boot: Minimal
support for booting U-Boot proper from SPL") and got lost with commit
089df18bfe9d ("lib: move hash CONFIG options to Kconfig").

Fixes: 089df18bfe9d ("lib: move hash CONFIG options to Kconfig")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/lib/Makefile b/lib/Makefile
index 51eba80..15259d0 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -58,7 +58,7 @@
 obj-$(CONFIG_TPM_V2) += tpm-v2.o
 endif
 
-obj-$(CONFIG_RSA) += rsa/
+obj-$(CONFIG_$(SPL_)RSA) += rsa/
 obj-$(CONFIG_SHA1) += sha1.o
 obj-$(CONFIG_SHA256) += sha256.o