Convert ARCH_OMAP2PLUS boards' CONFIG_SYS_TEXT_BASE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_TEXT_BASE

The includes, whitelist, etc. were left for now but I don't get any
build errors or warnings on the omap3_logic_defconfig or
am3517_evm_defconfig builds I tried.

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 803dd38..1d07f1b 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 # CONFIG_SYS_THUMB_BUILD is not set
 CONFIG_ARCH_OMAP2PLUS=y
+CONFIG_SYS_TEXT_BASE=0x80100000
 # CONFIG_SPL_GPIO_SUPPORT is not set
 CONFIG_TARGET_AM3517_EVM=y
 CONFIG_SYS_EXTRA_OPTIONS="NAND"