spl: binman: Fix use of undeclared u_boot_any symbols

Some SPL functions directly use the binman 'u_boot_any' symbols to get
U-Boot's binman image position. These symbols are declared by the
SPL/TPL_BINMAN_SYMBOLS configs, but they are accessed by macros defined
by just CONFIG_BINMAN. So when BINMAN is enabled and BINMAN_SYMBOLS is
disabled, the code tries to use undeclared symbols and we get an error.

Therefore, any use of 'u_boot_any' symbols in the code is an implicit
dependency on SPL/TPL_BINMAN_SYMBOLS. However, in the current uses
they are meant to be the next phase's values, where that happens to be
U-Boot. In the meantime, helper funcions spl_get_image_pos/size() were
introduced to get these values.

Convert all uses of u_boot_any symbols to these functions, so we only
access these symbols at one place. Make sure they will not use these
symbols when the BINMAN_SYMBOLS configs are disabled, by returning early
in those cases.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>