bootutil: Allow larger minimum flash write

[kristine.jassmann@renesas.com: Allow larger minimum flash write]
[michael.thomas@renesas.com: Add changes for 1.8]
[michael.thomas@renesas.com: Add magic alignment fix]
[gustavo.nihei@espressif.com: bootutil: Address issues from PR 949]

Co-authored-by: Kristine Jassmann <kristine.jassmann@renesas.com>
Co-authored-by: Michael Thomas <michael.thomas@renesas.com>
Co-authored-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>
Signed-off-by: Kristine Jassmann <kristine.jassmann@renesas.com>
Signed-off-by: Michael Thomas <michael.thomas@renesas.com>
Signed-off-by: Gustavo Henrique Nihei <gustavo.nihei@espressif.com>