am335x, shc: kconfig: Fix misspelled choice default

There is no EMMC symbol in the "enable different boot versions for the
shc board" choice. SHC_EMMC was probably intended.

No functional changes. Kconfig choices fall back on using the first
(visible) symbol in the choice as the default if the default symbol is
not visible.

Discovered in Kconfiglib (https://github.com/ulfalizer/Kconfiglib),
which prints the following warning:

	warning: the default selection EMMC (undefined) of <choice> (defined at board/bosch/shc/Kconfig:15) is not contained in the choice

I've added a corresponding warning to the C tools too, which is
currently in linux-next: https://patchwork.kernel.org/patch/9983667/



Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>