binman: Add a new "skip-at-start" property in Section class

binman: Add a new "skip-at-start" property in Section class

Currently binman calculates '_skip_at_start' based on 'end-at-4gb'
property and it is used for x86 images.

For PowerPC mpc85xx based CPU, CONFIG_SYS_TEXT_BASE is the entry
offset of the first entry. It can be 0xeff40000 or 0xfff40000 for
nor flash boot, 0x201000 for sd boot etc, so "_skip_at_start"
should be set to CONFIG_SYS_TEXT_BASE.

'end-at-4gb' property is not applicable where CONFIG_SYS_TEXT_BASE +
Image size != 4gb.

Add new property 'skip-at-start' in Section class so that
'_skip_at_start' can be calculated either based on 'end-at-4gb'
or based on "skip-at-start".

Add a test case to check that 'skip-at-start' and 'end-at-4gb'
property can't be used together.

Signed-off-by: Jagdish Gediya <jagdish.gediya@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>