Move existing use of CONFIG_SPL_RSA to Kconfig

A few boards define this in a header file which is incorrect. It means that
Kconfig options that rely on this cannot be used. Move it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/C29XPCIE_SPIFLASH_SECBOOT_defconfig b/configs/C29XPCIE_SPIFLASH_SECBOOT_defconfig
index 407093c..7f4189e 100644
--- a/configs/C29XPCIE_SPIFLASH_SECBOOT_defconfig
+++ b/configs/C29XPCIE_SPIFLASH_SECBOOT_defconfig
@@ -22,4 +22,5 @@
 CONFIG_SYS_NS16550=y
 CONFIG_FSL_ESPI=y
 CONFIG_RSA=y
+CONFIG_SPL_RSA=y
 CONFIG_OF_LIBFDT=y