spl: armv7m: to keep ARM v7M in thumb mode before booting next image

On ARM v7M, the processor will return to ARM mode when executing blx
instruction with bit 0 of the address == 0. Always set it to 1 to stay in thumb
mode.

At present, it is applied only for raw U-Boot. This patch moves it to just
before booting next image. This way armv7m will be in thumb mode for any image
like raw or image with header like zImage or standard U-Boot.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>